Time-based access of a memory cell

ABSTRACT

Methods, systems, and devices for time-based access of memory cells in a memory array are described herein. During a sense portion of a read operation, a selected memory cell may be charged to a predetermined voltage level. A logic state stored on the selected memory cell may be identified based on a duration between the beginning of the charging and when selected memory cell reaches the predetermined voltage level. In some examples, time-varying signals may be used to indicate the logic state based on the duration of the charging. The duration of the charging may be based on a polarization state of the selected memory cell, a dielectric charge state of the selected state, or both a polarization state and a dielectric charge state of the selected memory cell.

CROSS REFERENCE

The present application for patent is a continuation of U.S. patentapplication Ser. No. 16/032,398 by Di Vincenzo et al., entitled“Time-Based Access of a Memory Cell,” filed Jul. 11, 2018, which is adivisional of U.S. patent application Ser. No. 15/619,158 by Di Vincenzoet al., entitled “Time-Based Access of a Memory Cell,” filed Jun. 9,2017, which is related to co-pending U.S. patent application Ser. No.15/619,163 by Di Vincenzo, entitled “Time-Based Access of a MemoryCell,” filed Jun. 9, 2017, assigned to the assignee hereof, and each ofwhich is expressly incorporated by reference herein.

BACKGROUND

The following relates generally to time-based access of a memory celland more specifically to time-based sensing of a logic state of thememory cell.

Memory devices are widely used to store information in variouselectronic devices such as computers, wireless communication devices,cameras, digital displays, and the like. Information is stored byprogramming different states of a memory device. For example, binarydevices have two states, often denoted by a logic “1” or a logic “0.” Inother systems, more than two states may be stored. To access the storedinformation, a component of the electronic device may read, or sense,the stored state in the memory device. To store information, a componentof the electronic device may write, or program, the state in the memorydevice.

Various types of memory devices exist, including magnetic hard disks,random access memory (RAM), read only memory (ROM), dynamic RAM (DRAM),synchronous dynamic RAM (SDRAM), ferroelectric RAM (FeRAM), magnetic RAM(MRAM), resistive RAM (RRAM), flash memory, phase change memory (PCM),and others. Memory devices may be volatile or non-volatile. Non-volatilememory, e.g., FeRAM, may maintain their stored logic state for extendedperiods of time even in the absence of an external power source.Volatile memory devices, e.g., DRAM, may lose their stored state overtime unless they are periodically refreshed by an external power source.FeRAM may use similar device architectures as volatile memory but mayhave non-volatile properties due to the use of a ferroelectric capacitoras a storage device. FeRAM devices may thus have improved performancecompared to other non-volatile and volatile memory devices.

Improving memory devices, generally, may include increasing memory celldensity, increasing read/write speeds, increasing reliability,increasing data retention, reducing power consumption, or reducingmanufacturing costs, among other metrics. It may also be beneficial toemploy memory cells capable of storing more than two logic states.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a memory array that supports time-basedaccess of a memory cell in accordance with embodiments of the presentdisclosure.

FIG. 2 illustrates an example of a circuit that supports time-basedaccess of a memory cell in accordance with embodiments of the presentdisclosure.

FIG. 3 illustrates an example of hysteresis curves that supporttime-based access of a memory cell in accordance with embodiments of thepresent disclosure.

FIG. 4 illustrates an example of state diagrams that support time-basedaccess of a memory cell in accordance with embodiments of the presentdisclosure.

FIG. 5 illustrates an example of a timing diagram that supportstime-based access of a memory cell in accordance with embodiments of thepresent disclosure.

FIG. 6 illustrates an example of a circuit that supports time-basedaccess of a memory cell in accordance with embodiments of the presentdisclosure.

FIG. 7 illustrates an example of a timing diagram that supportstime-based access of a memory cell in accordance with embodiments of thepresent disclosure.

FIG. 8 illustrates an example of a timing diagram that supportstime-based access of a memory cell in accordance with embodiments of thepresent disclosure.

FIG. 9 illustrates an example of a timing diagram that supportstime-based access of a memory cell in accordance with embodiments of thepresent disclosure.

FIGS. 10 through 11 show block diagrams of a device that supportstime-based access of a memory cell in accordance with embodiments of thepresent disclosure.

FIG. 12 illustrates a block diagram of a system including a memorycontroller that supports time-based access of a memory cell inaccordance with embodiments of the present disclosure.

FIGS. 13 through 14 illustrate methods for time-based access of a memorycell in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION

Methods, systems, and devices for time-based or time-resolved access ofmemory cells in a memory array are described herein. This may allow acell to store more than two logic states and thus more than two bits.The described techniques may also provide other benefits as describedherein.

By way of example, memory devices generally use voltages to distinguishbetween logic states stored on memory cells. For example, during a readoperation of a memory cell, a memory controller may cause the memorycell to discharge a charge or a voltage onto an access line. The memorycontroller may identify the logic state stored on the memory cell basedon a comparison between the voltage of the access line and a referencevoltage. In some examples, using voltage levels to distinguish betweenlogic states of memory cells may limit how many unique logic states maybe stored on a memory cell.

During a sense portion of a read operation, a selected memory cell maybe charged to a predetermined voltage level. A logic state stored on theselected memory cell may be identified based on a duration between thebeginning of the charging and when selected memory cell reaches thepredetermined voltage level. In some examples, time-varying signals maybe used to indicate the logic state based on the duration of thecharging. In some examples, the duration of the charging may be based ona polarization state of the selected memory cell, a dielectric chargestate of the selected state, or both a polarization state and adielectric charge state of the selected memory cell.

A number of advantages may be realized using time-based sensingtechniques during a read operation. In some examples, logic states maybe distinguishable using time-based techniques that are notdistinguishable using voltage-based techniques. In some examples, apre-existing memory cell may be configured to store more logic statesthan is possible using voltage-based sensing techniques. Additionaladvantages of the techniques, systems, and devices described herein maybe apparent based on the features described below.

Features of the disclosure introduced above are further described belowin the context of FIGS. 1-12. These and other features of the disclosureare further illustrated by and described with reference to apparatusdiagrams, system diagrams, and flowcharts that relate to time-basedaccess of a memory cell.

FIG. 1 illustrates an example memory array 100 in accordance withvarious embodiments of the present disclosure. Memory array 100 may alsobe referred to as an electronic memory apparatus. Memory array 100includes memory cells 105 that are programmable to store differentstates. Each memory cell 105 may be programmable to store two states,denoted as a logic 0 and a logic 1. In some cases, memory cell 105 isconfigured to store more than two logic states. A memory cell 105 maystore a charge representative of the programmable states in a capacitor;for example, a charged and uncharged capacitor may represent two logicstates, respectively. DRAM architectures may commonly use such a design,and the capacitor employed may include a dielectric material with linearor para-electric electric polarization properties as the insulator. Bycontrast, a ferroelectric memory cell may include a capacitor with aferroelectric as the insulating material. Different levels of charge ofa ferroelectric capacitor may represent different logic states.Ferroelectric materials have non-linear polarization properties; somedetails and advantages of a ferroelectric memory cell 105 are discussedbelow.

Operations such as reading and writing may be performed on memory cells105 by activating or selecting access line 110 and digit line 115.Access lines 110 may also be known as word lines 110, and bit lines 115may also be known digit lines 115. References to word lines and bitlines, or their analogues, are interchangeable without loss ofunderstanding or operation. Activating or selecting a word line 110 or adigit line 115 may include applying a voltage to the respective line.Word lines 110 and digit lines 115 may be made of conductive materialssuch as metals (e.g., copper (Cu), aluminum (Al), gold (Au), tungsten(W), etc.), metal alloys, carbon, conductively-doped semiconductors, orother conductive materials, alloys, compounds, or the like. In someexamples, other lines (e.g., plates lines—not shown in FIG. 1) also maybe present.

According to the example of FIG. 1, each row of memory cells 105 isconnected to a single word line 110, and each column of memory cells 105is connected to a single digit line 115. By activating one word line 110and one digit line 115 (e.g., applying a voltage to the word line 110 ordigit line 115), a single memory cell 105 may be accessed at theirintersection. Accessing the memory cell 105 may include reading orwriting the memory cell 105. The intersection of a word line 110 anddigit line 115 may be referred to as an address of a memory cell. Insome instances, the read operation of a memory cell 105 may betime-based. Meaning, for example, that the logic state of the memorycell 105 may be determined based on a duration to satisfy a conditionrather than a voltage level satisfying a condition or a threshold. Forexample, the memory controller 140 may determine the logic state of amemory cell 105 based on a duration it takes for the digit line to becharged to a predetermined voltage. In some examples, the memory cells105 may be configured as a volatile memory cell, a non-volatile memorycell, or a partly volatile and partly non-volatile memory cell.

In some architectures, the logic storing device of a cell, e.g., acapacitor, may be electrically isolated from the digit line by aselection component. The word line 110 may be connected to and maycontrol the selection component. For example, the selection componentmay be a transistor and the word line 110 may be connected to the gateof the transistor. Activating the word line 110 results in an electricalconnection or closed circuit between the capacitor of a memory cell 105and its corresponding digit line 115. The digit line may then beaccessed to either read or write the memory cell 105.

Accessing memory cells 105 may be controlled through a row decoder 120and a column decoder 130. For example, a row decoder 120 may receive arow address from the memory controller 140 and activate the appropriateword line 110 based on the received row address. Similarly, a columndecoder 130 receives a column address from the memory controller 140 andactivates the appropriate digit line 115. For example, memory array 100may include multiple word lines 110, labeled WL_1 through WL_M, andmultiple digit lines 115, labeled DL 1 through DL N, where M and Ndepend on the array size. Thus, by activating a word line 110 and adigit line 115, e.g., WL_2 and DL_3, the memory cell 105 at theirintersection may be accessed.

Upon accessing, a memory cell 105 may be read, or sensed, by sensecomponent 125 to determine the stored state of the memory cell 105. Forexample, after accessing the memory cell 105, the ferroelectriccapacitor of memory cell 105 may discharge onto its corresponding digitline 115. Discharging the ferroelectric capacitor may result frombiasing, or applying a voltage, to the ferroelectric capacitor. Thedischarging may cause a change in the voltage of the digit line 115,which sense component 125 may compare to a reference voltage (not shown)in order to determine the stored state of the memory cell 105. Forexample, if digit line 115 has a higher voltage than the referencevoltage, then sense component 125 may determine that the stored state inmemory cell 105 was a logic 1 and vice versa. Sense component 125 mayinclude various transistors or amplifiers in order to detect and amplifya difference in the signals, which may be referred to as latching. Thedetected logic state of memory cell 105 may then be output throughcolumn decoder 130 as output 135. In some cases, sense component 125 maybe part of a column decoder 130 or row decoder 120. Or, sense component125 may be connected to or in electronic communication with columndecoder 130 or row decoder 120. In some instances, the sense component125 may be configured to activate a latch based on the duration for thedigit line to be charged to the predetermined voltage. The logic stateof the associated memory cell may be determined based on a value of atime-varying signal at the time the latch is activated. In someexamples, the sense component 125 may include a decoder system 145.During an access operation (e.g., a read operation or a writeoperation), a plurality of digit lines 115 may be selected. The decodersystem 145 may be configured to coordinate an access operation whenmultiple digit lines 115 are selected as part the access operation. Insome instances, a pre-decoding system (not shown) may be positionedbetween the digit lines 115 and the sense component 125 to performsimilar functions as the decoder system 145.

A memory cell 105 may be set, or written, by similarly activating therelevant word line 110 and digit line 115—i.e., a logic value may bestored in the memory cell 105. Column decoder 130 or row decoder 120 mayaccept data, for example input/output 135, to be written to the memorycells 105. A ferroelectric memory cell 105 may be written by applying avoltage across the ferroelectric capacitor. This process is discussed inmore detail below. In some instances, the memory cell 105 may beconfigured to store more than two logic states. For example, aferroelectric memory cell may be configured to store at least threestates by storing both a polarization state and a dielectric chargestate. Using both of those states at least one of three logic states ofthe ferroelectric memory cell may be determined. In some cases, thepolarization state and the dielectric charge state of the ferroelectricmemory cell may be used to identify four or more logic states that maybe stored on the memory cell. For example, the ferroelectric memory cellmay be configured to store a plurality of polarization states and aplurality dielectric charge states. Various combinations of thepolarization states and the dielectric charge states may define aplurality of logic states of the ferroelectric memory cell. In otherexamples, a dielectric memory cell may be configured to store aplurality of different dielectric charge states and thereby define morethan two logic states. In other examples, a ferroelectric memory cellmay be configured to store a plurality of different polarization statesand thereby define more than two logic states.

In some memory architectures, accessing the memory cell 105 may degradeor destroy the stored logic state and re-write or refresh operations maybe performed to return the original logic state to memory cell 105. InDRAM, for example, the capacitor may be partially or completelydischarged during a sense operation, corrupting the stored logic state.So the logic state may be re-written after a sense operation.Additionally, activating a single word line 110 may result in thedischarge of all memory cells in the row; thus, several or all memorycells 105 in the row may need to be re-written. In some instances, aferroelectric memory cell may be configured to store both polarizationstates and dielectric charge states. As such, access operations, such aswrite operations, may be modified to store both a polarization state anda dielectric charge state on the ferroelectric memory cell.

Some memory architectures, including DRAM, may lose their stored stateover time unless they are periodically refreshed by an external powersource. For example, a charged capacitor may become discharged over timethrough leakage currents, resulting in the loss of the storedinformation. The refresh rate of these so-called volatile memory devicesmay be relatively high, e.g., tens of refresh operations per second forDRAM arrays, which may result in significant power consumption. Withincreasingly larger memory arrays, increased power consumption mayinhibit the deployment or operation of memory arrays (e.g., powersupplies, heat generation, material limits, etc.), especially for mobiledevices that rely on a finite power source, such as a battery. Asdiscussed below, ferroelectric memory cells 105 may have beneficialproperties that may result in improved performance relative to othermemory architectures.

The memory controller 140 may control the operation (e.g., read, write,re-write, refresh, discharge, etc.) of memory cells 105 through thevarious components, for example, row decoder 120, column decoder 130,and sense component 125. In some cases, one or more of the row decoder120, column decoder 130, and sense component 125 may be co-located withthe memory controller 140. Memory controller 140 may generate row andcolumn address signals in order to activate the desired word line 110and digit line 115. Memory controller 140 may also generate and controlvarious voltages or currents used during the operation of memory array100. For example, it may apply discharge voltages to a word line 110 ordigit line 115 after accessing one or more memory cells 105. In general,the amplitude, shape, or duration of an applied voltage or currentdiscussed herein may be adjusted or varied and may be different for thevarious operations discussed in operating memory array 100. Furthermore,one, multiple, or all memory cells 105 within memory array 100 may beaccessed simultaneously; for example, multiple or all cells of memoryarray 100 may be accessed simultaneously during a reset operation inwhich all memory cells 105, or a group of memory cells 105, are set to asingle logic state. As is discussed in more detail below, accessoperations (e.g., read operation or write operation) performed by thememory controller 140 may be modified to account for time-based sensingand/or multiple logic states being stored on a memory cell 105.

FIG. 2 illustrates an example circuit 200 in accordance with variousembodiments of the present disclosure. Circuit 200 includes a memorycell 105-a, word line 110-a, digit line 115-a, and sense component125-a, which may be examples of a memory cell 105, word line 110, digitline 115, and sense component 125, respectively, as described withreference to FIG. 1. Memory cell 105-a may include a logic storagecomponent, such as capacitor 205 that has a first plate, cell plate 230,and a second plate, cell bottom 215. Cell plate 230 and cell bottom 215may be capacitively coupled through a ferroelectric material positionedbetween them. The orientation of cell plate 230 and cell bottom 215 maybe flipped without changing the operation of memory cell 105-a. Circuit200 also includes selection component 220 and reference line 225. Cellplate 230 may be accessed via plate line 210 and cell bottom 215 may beaccessed via digit line 115-a. As described above, various states may bestored by charging or discharging capacitor 205. In some cases, the cellbottom 215 (or the cell plate 230 as the case may be) may cooperate withthe selection component 220 to form a middle electrode 235. In someinstances, the middle electrode 235 may store a charge. In someexamples, the charge stored on the middle electrode 235 may contribute,at least in part, to the dielectric charge state of the memory cell105-a.

The stored state of capacitor 205 may be read or sensed by operatingvarious elements represented in circuit 200. Capacitor 205 may be inelectronic communication with digit line 115-a. For example, capacitor205 can be isolated from digit line 115-a when selection component 220is deactivated, and capacitor 205 can be connected to digit line 115-awhen selection component 220 is activated. Activating selectioncomponent 220 may be referred to as selecting memory cell 105-a. In somecases, selection component 220 is a transistor and its operation iscontrolled by applying a voltage to the transistor gate, where thevoltage magnitude is greater than the threshold magnitude of thetransistor. Word line 110-a may activate selection component 220; forexample, a voltage applied to word line 110-a is applied to thetransistor gate, connecting capacitor 205 with digit line 115-a. Asdiscussed in more detail below, the logic state of a memory cell 105-amay be determined based on a duration of time to charge the memory cell105. Such a time-resolved sensing may enable the memory cell 105 tostore additional logic states as compared to voltage-resolved sensing.

In other examples, the positions of selection component 220 andcapacitor 205 may be switched, such that selection component 220 isconnected between plate line 210 and cell plate 230 and such thatcapacitor 205 is between digit line 115-a and the other terminal ofselection component 220. In this embodiment, selection component 220 mayremain in electronic communication with digit line 115-a throughcapacitor 205. This configuration may be associated with alternativetiming and biasing for read and write operations.

Due to the ferroelectric material between the plates of capacitor 205,and as discussed in more detail below, capacitor 205 may not dischargeupon connection to digit line 115-a. In one scheme, to sense the logicstate stored by ferroelectric capacitor 205, word line 110-a may bebiased to select memory cell 105-a and a voltage may be applied to plateline 210. In some cases, digit line 115-a is virtually grounded and thenisolated from the virtual ground, which may be referred to as“floating,” prior to biasing plate line 210 and word line 110-a. Biasingplate line 210 may result in a voltage difference (e.g., plate line 210voltage minus digit line 115-a voltage) across capacitor 205. Thevoltage difference may yield a change in the stored charge on capacitor205, where the magnitude of the change in stored charge may depend onthe initial state of capacitor 205—e.g., whether the initial statestored a logic 1 or a logic 0. This may cause a change in the voltage ofdigit line 115-a based on the charge stored on capacitor 205. Operationof memory cell 105-a by varying the voltage to cell plate 230 may bereferred to as “moving cell plate.” In some instances, the digit line115-a may be charged to a predetermined voltage level during a readoperation. A duration to perform such charging may be based on the logicstate stored on the memory cell 105-a.

The change in voltage of digit line 115-a may depend on its intrinsiccapacitance. That is, as charge flows through digit line 115-a, somefinite charge may be stored in digit line 115-a and the resultingvoltage depends on the intrinsic capacitance. The intrinsic capacitancemay depend on physical characteristics, including the dimensions, ofdigit line 115-a. Digit line 115-a may connect many memory cells 105 sodigit line 115-a may have a length that results in a non-negligiblecapacitance (e.g., on the order of picofarads (pF)). The resultingvoltage of digit line 115-a may then be compared to a reference (e.g., avoltage of reference line 225) by sense component 125-a in order todetermine the stored logic state in memory cell 105-a. Other sensingprocesses may be used. In some examples, determination of the storedlogic state may be based, at least in part, on a duration of time tocharge the digit line to a voltage level.

Sense component 125-a may include various transistors or amplifiers todetect and amplify a difference in signals, which may be referred to aslatching. Sense component 125-a may include a sense amplifier thatreceives and compares the voltage of digit line 115-a and reference line225, which may be a reference voltage. The sense amplifier output may bedriven to the higher (e.g., a positive) or lower (e.g., negative orground) supply voltage based on the comparison. For instance, if digitline 115-a has a higher voltage than reference line 225, then the senseamplifier output may be driven to a positive supply voltage. In somecases, the sense amplifier may additionally drive digit line 115-a tothe supply voltage. Sense component 125-a may then latch the output ofthe sense amplifier and/or the voltage of digit line 115-a, which may beused to determine the stored state in memory cell 105-a, e.g., logic 1.Alternatively, if digit line 115-a has a lower voltage than referenceline 225, the sense amplifier output may be driven to a negative orground voltage. Sense component 125-a may similarly latch the senseamplifier output to determine the stored state in memory cell 105-a,e.g., logic 0. In some examples, determination of the state stored inthe memory cell may depend, at least in part, on a duration of time tocharge to a voltage level. The latched logic state of memory cell 105-amay then be output, for example, through column decoder 130 as output135 with reference to FIG. 1. In some instances, the sense component125-a may be configured to determine when the digit line 115-a ischarged to a predetermined voltage level. In some examples, the sensecomponent 125-a may activate a latch based on determining that the digitline has been charged to the predetermined voltage level. A logic stateof the memory cell 105-a may be based on a value of a time-varyingsignal of the latch at the time the latch is activated.

To write memory cell 105-a, a voltage may be applied across capacitor205. Various methods may be used. In one example, selection component220 may be activated through word line 110-a in order to electricallyconnect capacitor 205 to digit line 115-a. A voltage may be appliedacross capacitor 205 by controlling the voltage of cell plate 230(through plate line 210) and cell bottom 215 (through digit line 115-a).To write a logic 0, cell plate 230 may be taken high, that is, apositive voltage may be applied to plate line 210, and cell bottom 215may be taken low, e.g., virtually grounding or applying a negativevoltage to digit line 115-a. The opposite process is performed to writea logic 1, where cell plate 230 is taken low and cell bottom 215 istaken high. In some examples, the write procedure may be modified toaccount for multiple bits being stored in a single memory cell.

FIG. 3 illustrates an example of non-linear electrical properties withhysteresis curves 300-a and 300-b for a ferroelectric memory cell thatis operated in accordance with various embodiments of the presentdisclosure. Hysteresis curves 300-a and 300-b illustrate an exampleferroelectric memory cell writing and reading process, respectively.Hysteresis curves 300-a and 300-b depict the charge, Q, stored on aferroelectric capacitor (e.g., capacitor 205 of FIG. 2) as a function ofa voltage difference, V.

A ferroelectric material is characterized by a spontaneous electricpolarization, i.e., it maintains a non-zero electric polarization in theabsence of an electric field. Example ferroelectric materials includebarium titanate (BaTiO₃), lead titanate (PbTiO₃), lead zirconiumtitanate (PZT), and strontium bismuth tantalate (SBT). The ferroelectriccapacitors described herein may include these or other ferroelectricmaterials. Electric polarization within a ferroelectric capacitorresults in a net charge at the ferroelectric material's surface andattracts opposite charge through the capacitor terminals. Thus, chargeis stored at the interface of the ferroelectric material and thecapacitor terminals. Because the electric polarization may be maintainedin the absence of an externally applied electric field for relativelylong times, even indefinitely, charge leakage may be significantlydecreased as compared with, for example, capacitors employed in DRAMarrays. This may reduce the need to perform refresh operations asdescribed above for some DRAM architectures.

Hysteresis curves 300-a and 300-b may be understood from the perspectiveof a single terminal of a capacitor. By way of example, if theferroelectric material has a negative polarization, positive chargeaccumulates at the terminal. Likewise, if the ferroelectric material hasa positive polarization, negative charge accumulates at the terminal.Additionally, it should be understood that the voltages in hysteresiscurves 300-a and 300-b represent a voltage difference across thecapacitor and are directional. For example, a positive voltage may berealized by applying a positive voltage to the terminal in question(e.g., a cell plate 230) and maintaining the second terminal (e.g., acell bottom 215) at ground (or approximately zero volts (0V)). Anegative voltage may be applied by maintaining the terminal in questionat ground and applying a positive voltage to the second terminal—i.e.,positive voltages may be applied to negatively polarize the terminal inquestion. Similarly, two positive voltages, two negative voltages, orany combination of positive and negative voltages may be applied to theappropriate capacitor terminals to generate the voltage difference shownin hysteresis curves 300-a and 300-b.

As depicted in hysteresis curve 300-a, the ferroelectric material maymaintain a positive or negative polarization with a zero voltagedifference, resulting, in some cases, in two possible memory states:memory state 305 (State B) and memory state 310 (State C). According tothe example of FIG. 3, memory state 305 (State B) represents a logic 0and memory state 310 (State C) represents a logic 1. In some examples,the logic values of the respective memory states may be reversed toaccommodate other schemes for operating a memory cell.

A logic 0 or 1 may be written to the memory cell by controlling theelectric polarization of the ferroelectric material by applying voltage.For example, applying a net positive biasing voltage 315 across thecapacitor results in charge accumulation until a memory state 340 (StateA) is reached. Upon removing the biasing voltage 315, the memory state340 (State A) follows path 320 until it reaches the memory state 305(State B) at zero voltage. Similarly, memory state 310 (State C) iswritten by applying a net negative biasing voltage 325, which results ina memory state 345 (State D). After removing negative voltage 325,memory state 345 (State D) follows path 330 until it reaches memorystate 310 (State C) at zero voltage. Memory states 340 (State A) and 345(State D) may also be referred to as the remnant polarization (Pr)values, i.e., the polarization (or charge) that remains upon removingthe external bias (e.g., voltage). The coercive voltage is the voltageat which the charge (or polarization) is zero.

To read, or sense, the stored state of the ferroelectric capacitor, avoltage may be applied across the capacitor. In response, the storedcharge, Q, changes, and the degree of the change depends on the initialcharge state—i.e., the final stored charge (Q) depends on whether memorystate 305-a or 310-a was initially stored. For example, hysteresis curve300-b illustrates two possible stored memory states 305-a and 310-a.Biasing voltage 335 may be applied across the capacitor as discussedwith reference to FIG. 2. In other cases, a fixed voltage may be appliedto the cell plate and, although depicted as a positive voltage, thebiasing voltage 335 may be negative. In response to the biasing voltage335, the memory state 305-a may follow path 350. Likewise, if memorystate 310-a was initially stored, then it follows path 355. The finalposition of memory state 360 and memory state 365 depend on a number offactors, including the specific sensing scheme and circuitry.

In some cases, the final memory state may depend on the intrinsiccapacitance of the digit line connected to the memory cell. For example,if the capacitor is electrically connected to the digit line and voltage335 is applied, the voltage of the digit line may rise due to itsintrinsic capacitance. So a voltage measured at a sense component maynot equal voltage 335 and instead may depend on the voltage of the digitline. The position of final memory states 360 and 365 on hysteresiscurve 300-b may thus depend on the capacitance of the digit line and maybe determined through a load-line analysis—i.e., memory states 360 and365 may be defined with respect to the digit line capacitance. As aresult, the voltage of the capacitor, voltage 370 or voltage 375, may bedifferent and may depend on the initial state of the capacitor.

By comparing the digit line voltage to a reference voltage, the initialstate of the capacitor may be determined. The digit line voltage may bethe difference between voltage 335 and the final voltage across thecapacitor, voltage 370 or voltage 375—i.e., (voltage 335—voltage 370) or(voltage 335—voltage 375). A reference voltage may be generated suchthat its magnitude is between the two possible voltages of the twopossible digit line voltages in order to determine the stored logicstate—i.e., if the digit line voltage is higher or lower than thereference voltage. For example, the reference voltage may be an averageof the two quantities, (voltage 335—voltage 370) and (voltage335—voltage 375). Upon comparison by the sense component, the senseddigit line voltage may be determined to be higher or lower than thereference voltage, and the stored logic value of the ferroelectricmemory cell (i.e., a logic 0 or 1) may be determined. In some examples,the access procedures (e.g., read or write) of the memory cell may bemodified to account for multiple bits being stored in a single memorycell.

As discussed above, reading a memory cell that does not use aferroelectric capacitor may degrade or destroy the stored logic state. Aferroelectric memory cell, however, may maintain the initial logic stateafter a read operation. For example, if memory state 305-a is stored,the memory state may follow path 350 to memory state 360 during a readoperation and, after removing voltage 335, the charge state may returnto initial memory state 305-a by following path 350 in the oppositedirection.

In some instances, a ferroelectric memory cell may be configured tomaintain more than two memory states. In some examples, to maintain morethan two memory states, the ferroelectric memory cell may be configuredto store a polarization state (e.g., a stable state) and a dielectriccharge state (e.g., a volatile state). The polarization state may beassociated with properties of the ferroelectric material (i.e.,polarization of the cell) and the dielectric charge state may beassociated with voltage or charge stored on the capacitor. The pluralityof logic states of the memory cell may include a plurality of stablestates, a plurality of volatile states, or combinations thereof.

For example, a ferroelectric memory cell may be configured to store fourmemory states: memory state 305 (State B), memory state 310 (State C),memory state 340 (State A), and memory state 345 (State D). In someexamples, a logic value stored in the memory cell may be based on acombination of a polarization state (e.g., stable state) and adielectric charge (e.g., volatile state). In some examples, the numberof logic values that may be stored in a memory cell is based on a numberof possible combinations of polarization states and dielectric chargestates. The memory state 305 (State B) may be based on the memory cellhaving a positive polarization state and zero-value for the dielectriccharge state. The memory state 310 (State C) may be based on the memorycell having a negative polarization state and zero-value for thedielectric charge state. The memory state 340 (State A) may be based onthe memory cell having a positive polarization state and positivenon-zero voltage or charge for the dielectric charge state. The memorystate 345 (State D) may be based on the memory cell having a negativepolarization state and negative non-zero voltage or charge for thedielectric charge state.

To store both a polarization state and a dielectric charge state on amemory cell, various operations of a memory array may be altered. Forexample, during a write operation a memory controller may not dischargethe middle electrode of the memory cell. In such examples, the memorycell may maintain a non-zero dielectric charge state.

When a memory cell includes two memory states a single bit of logic maybe stored by the memory cell. However, when a memory cell include morethan two memory states, additional bits of logic may be stored by thememory cell. For example, if the memory cell includes four memorystates, two bits of logic may be stored on memory cell. It should beappreciated that additional memory states may be stored on the memorycell based on different combinations of polarization state values anddielectric charge state values (e.g., memory state 360 and memory state365).

FIG. 4 illustrates an example of state diagrams 400 that supporttime-based access of a memory cell in accordance with variousembodiments of the present disclosure. Time-based sensing may be used toidentify a plurality of different types of memory states of a memorycell. For example, as shown in state diagram 400-a, a memory cell mayinclude more than two memory states that are based on polarization. Inanother example, as shown in the state diagram 400-b, a memory cell mayinclude more than two memory states based on dielectric charges and, insome examples, memory states may be based on both polarization anddielectric charges.

State diagram 400-a illustrates memory states for a memory cell thatincludes a plurality of polarization memory states 405. The plurality ofpolarization memory states 405 may include a first memory state 410, asecond memory state 415, a third memory state 420, and a fourth memorystate 425. While only four memory states are depicted, a memory cell mayinclude any number of memory states including two memory states, threememory states, four memory states, etc. Each of the memory states 410,415, 420, 425 may be based on a polarization of a ferroelectric memorycell. In some examples, each memory state 410, 415, 420, 425 may includezero dielectric charge. The memory states 410, 415, 420, 425 may becharacterized by having a non-zero polarization value and a zerodielectric charge value.

To obtain the memory state 410, a biasing voltage may be applied to theferroelectric memory cell to move the memory cell to point 430 of thehysteresis curve. After the biasing voltage is released, theferroelectric memory cell may relax back to a zero dielectric chargestate at memory state 410. Similarly, the memory state 425 may beobtained by applying a negative biasing voltage to the ferroelectricmemory cell. To obtain the memory states 415, 420, the biasing voltagesand/or the timings of a write operation may be altered. For example, thememory cell may be biased to a point different from the point 435 on thehysteresis curve, and/or a sequence of negative and positive voltagesmay be applied. In some examples, the absolute value of the positive andnegative voltage may decrease during the sequence of programming pulses.As discussed in more detail with regard to FIG. 5, time-based sensingtechniques may be configured to distinguish between the memory states410, 415, 420, 425.

State diagram 400-b illustrates memory states for a memory cell thatinclude a plurality of dielectric charge memory states 440 and aplurality of combination polarization and dielectric charge memorystates 445. The dielectric charge memory states 440 may include a firstmemory state 450, a second memory state 455, a third memory state 460,and a fourth memory state 465. While only four memory states aredepicted, a memory cell may include any number of memory statesincluding two memory states, three memory states, four memory states,etc. Each of the memory states may be based on a dielectric charge ofthe memory cell. In such examples, memory states with non-zerodielectric charge may have a corresponding voltage stored on thecapacitor. In some cases, a linear relationship (Q=CV) may exist betweencharge and voltage. Line 490 represents an example of the linearrelationship for dielectric charge states. In some examples, thedielectric charge is stored on a capacitor of the memory cell. In someexamples, the dielectric charge is stored on a middle electrode memorycell. In some examples, the dielectric charge is stored on both thecapacitor and a middle electrode of a memory cell. Each of the memorystates 450, 455, 460, 465 may be based on a dielectric charge of amemory cell. In some examples, dielectric charge states may beassociated with negative voltage and/or negative charge. In someexamples, each memory state 450, 455, 460, 465 may not be polarized. Assuch, either a dielectric capacitor (e.g., DRAM) or a ferroelectriccapacitor may be configured to store the memory states 450, 455, 460,465. In some examples, a ferroelectric memory cell may be used as a pureDRAM memory cell. In the example depicted in state diagram 440-b, theferroelectric memory cell may differ from a pure DRAM cell in that anon-zero polarization state is present; however, this difference may beneglected in some examples of multi-level (volatile) storage operationsof the memory cell.

To obtain one of the memory states 450, 455, 460, 465, voltageassociated with the particular memory state may be applied to acapacitor of the memory cell. Different voltages may be used to obtaindifferent memory states. In some examples, after applying the voltageassociated to a particular memory state, the memory cell may bedeselected (e.g., the capacitor may be isolated from the digit line).

The combination memory states 445 may include a first memory state 470,a second memory state 475, a third memory state 480, and a fourth memorystate 485. While only four memory states are depicted, a memory cell mayinclude any number of memory states including two memory states, threememory states, four memory states, etc. Each of the memory states may bebased on a both a polarization and a dielectric charge of the memorycell. Some memory states either the polarization or the dielectriccharge may be a zero value.

To obtain one of the memory states 470, 475, 480, 485, one or morebiasing voltages associated with the particular memory state may beapplied to a capacitor of the memory cell. Different biasing voltagesand different timings may be used to obtain different memory states. Insome examples, after applying the voltage associated to a particularcombined memory state, the memory cell may be deselected (e.g., thecapacitor may be isolated from the digit line), so that the capacitorstores the dielectric charge accumulated onto it.

In some examples, the dielectric charge memory states 440 and thecombination memory states 445 may be associated with positive biasingvoltages. In such examples, a single current generator may be configuredto charge a digit line during a time-based read operation. In someexamples, the dielectric charge memory states 440 and the combinationmemory states 445 may be associated with both negative and positivebiasing voltages (e.g., memory states 305, 310, 340, 345). In some ofthose examples, additional components may be used to execute accessoperations on a memory cell.

As used herein, a memory state may refer to a state of a memory cell.For example, a memory state may include a polarization and a dielectriccharge. As used herein, a logic state may refer to digital logicassociated with a memory state of a memory cell. For example, a logicstate may include a logic ‘0,’ a logic ‘ 1,’ a logic ‘00,’ a logic ‘01,’a logic ‘10,’ a logic ‘11,’ etc. Logic states may be mapped to memorystates. In some examples, a one-to-one mapping exists between logicstates and memory states. As used herein, the term memory state may beused interchangeably with the term logic state. As such, in someexamples, a logic state may include a polarization state or a dielectriccharge state or combinations thereof.

FIG. 5 illustrates an example of a timing diagram 500 that supportstime-based access of a memory cell in accordance with variousembodiments of the present disclosure. In some instances, a readoperation of a memory cell may distinguish between logic states based ontime and durations. For example, after biasing a memory cell 105 and/orits associated digit line 115, a sense component may detect differentresponses based on the memory state of the memory cell. A logic statestored on a memory cell 105 (or a memory state of the memory cell 105)may be determined by detecting the duration between applying the biasingand when a voltage of the memory cell satisfies a voltage threshold 505.

The timing diagram 500 is associated with the memory states depicted anddescribed with relation to hysteresis curve 300-a of FIG. 3. As such,the timing diagram 500 may be associated with memory states of aferroelectric memory cell. However, it should be appreciated that othertiming diagrams are encompassed by this disclosure based on the memorystates of the memory cell. For example, different timing diagrams may beassociated with dielectric memory cells.

The illustrative timing diagram 500 includes a first response signal510, a second response signal 515, a third response signal 520, and afourth response signal 525. Each response signal may be associated withone of the memory states depicted in hysteresis curve 300-a. Forexample, the first response signal may be associated with a memory cellstoring a memory state A (e.g., memory state 340 (State A)). The secondresponse signal 515 may be associated with a memory cell storing amemory state B (e.g., memory state 305 (State B)). The third responsesignal 520 may be associated with a memory cell storing a memory state C(e.g., memory state 310 (State C)). The fourth response signal 525 maybe associated with a memory cell storing a memory state D (e.g., memorystate 345 (State D)).

During a read operation, a power source (e.g., a current generator) maycharge the memory cell 105 to a predetermined voltage level. Based onthe amount of time taken from the memory cell to reach a voltagethreshold 505 associated with the predetermined voltage level, a memorycontroller 140 may be configured to determine what memory state isstored on the memory cell 105. In some examples, the predeterminedvoltage level may be the biasing voltage used to obtain the states(e.g., voltage 315). In some examples, the predetermined voltage levelmay be the dielectric charge voltage associated with the memory state A.

The duration needed to satisfy the voltage threshold 505 during a readoperation may be based on the memory state of the memory cell. At timet0, a voltage or a current may be applied to the memory cell to chargethe memory cell to the predetermined voltage level. At time t1, thefirst response signal 510 associated with the memory state A of a memorycell satisfies the voltage threshold 505. A duration 530 defined betweentime t0 and time t1 may be the duration used to determine whether thememory cell 105 is in the memory state A. In some examples, the memorystate A starts at a positive voltage 535 because of the dielectriccharge of the memory cell in the memory state A. In some examples, thepredetermined voltage level to which the memory cell is charged is basedon the positive voltage 535 associated with the memory state A. In someexamples, the voltage threshold 505 is based on the positive voltage 535associated with the memory state A. In such examples, the duration 530may be quite small due to this relationship. In some examples, theduration 530 may be zero because the voltage threshold 505 is set to beless than the positive voltage 535.

When the memory cell is charged to the predetermined positive voltagelevel, the memory state A of the memory cell may advance along ahysteresis curve following path 350, as shown in hysteresis curve 300-b.Because the memory state A is positioned so close to the predeterminedpositive voltage level, the duration 530 may be small. In some examples,the duration 530 may be about zero nanoseconds.

At time t2, the second response signal 515 associated with the memorystate B of the memory cell 105 may satisfy the voltage threshold 505. Aduration 540 defined between time t0 and time t2 may be the durationused to determine whether the memory cell 105 is in the memory state B.In some examples, the memory state B starts at a zero voltage 545because the memory cell storing the memory state B does not include anydielectric charge.

When the memory cell is charged to the predetermined positive voltagelevel, the memory state B of the memory cell may advance along ahysteresis curve following path 350, as shown in hysteresis curve 300-b.The duration 540 may be based at least in part on a length of thehysteresis curve the memory state B may travel before reaching thepredetermined positive voltage level. In some examples, the duration 540may be based at least in part on a constant current level used to chargethe memory cell.

At time t3, the third response signal 520 associated with the memorystate C of the memory cell 105 may satisfy the voltage threshold 505. Aduration 550 defined between time t0 and time t3 may be the durationused to determine whether the memory cell 105 is in the memory state C.In some examples, the memory state C starts at the zero voltage 545because the memory cell storing the memory state C does not include anydielectric charge.

When the memory cell is charged to the predetermined positive voltagelevel, the memory state C of the memory cell may advance along ahysteresis curve following path 355, as shown in hysteresis curve 300-b.The duration 550 may be based at least in part on a length of thehysteresis curve the memory state C may travel before reaching thepredetermined positive voltage level. In some examples, the duration 550may be based at least in part on a constant current level used to chargethe memory cell. In some examples, the starting voltages for both thesecond response signal 515 and the third response signal 520 are thesame but the duration 540 is different than the duration 550. Such aphenomenon may be the result of different polarization states. Thememory state C may travel a different path along the hysteresis curve toreach the predetermined voltage level, and as such it may take longerfor the digit line 115 to satisfy the voltage threshold 505 (e.g.,duration 550 may be longer than duration 540). In some examples, if thememory cell is in memory state 310 (State C), some of the chargeinjected in the memory cell may be used to flip its polarization stateand some of the charge may be used to charge the memory cell so thatmore charge (or time) may be used to satisfy the voltage threshold 505with respond to a memory cell in memory state 305 (State B).

At time t4, the fourth response signal 525 associated with the memorystate D of the memory cell 105 may satisfy the voltage threshold 505. Aduration 555 defined between time t0 and time t4 may be the durationused to determine whether the memory cell 105 is in the memory state D.In some examples, the memory state D starts at a negative voltage 560because the memory cell storing the memory state D does includes anegative dielectric charge.

When the memory cell is charged to the predetermined positive voltagelevel, the memory state D of the memory cell may advance along ahysteresis curve following path 355, as shown in hysteresis curve 300-band continue along the hysteresis curve to the predetermined memorystate. The duration 555 may be based at least in part on a length of thehysteresis curve the memory state D may travel before reaching thepredetermined positive voltage level. In some examples, the duration 555may be based at least in part on a constant current level used to chargethe memory cell.

In some instances, a memory controller 140 may be configured todetermine the logic state stored on a memory cell 105 after time t3. Forexample, if the voltage threshold 505 has not been satisfied by time t3,the memory controller 140 may determine by inference that the memorycell 105 is in a memory state D. To make such inferences, in someexamples, a memory controller 140 define a time threshold 565 for a readoperation. If the voltage threshold 505 is not satisfied when the timethreshold 565 is satisfied, the memory controller 140 may determine (byinference) that the memory cell 105 is in a specific memory state. Inthe illustrative example of FIG. 5, the time threshold 565 may be setabout at time t3 or shortly after time t3 and the time threshold 565 maybe used to determine (by inference) that the memory cell 105 is in thememory state D. The use of a time threshold 565 may reduce the amount oftotal time used to perform a read operation. For example, during a readoperation, a memory controller 140 may reduce the amount of time itattempts to detect whether the voltage threshold 505 is satisfied basedon the time threshold 565.

In some examples, a response signal (not shown in FIG. 5) may exist forany given memory state (e.g., for any given point in the Q-V diagramrepresenting all possible combinations of polarization states anddielectric charge state of a memory cell). For any given memory state,the response signal may have one or more linear portions associated withdielectric charging (and/or discharging) the memory cell capacitor andone or more other portions (typically with a slower slope) correspondingto a modification of the polarization of the capacitor. The duration ofeach portion (either dielectric charging or polarization) may be basedat least in part on the initial memory state (e.g., on the combinationof the polarization and the dielectric charge stored on the capacitor)and/or on the current used to charge the memory cell to thepredetermined voltage level.

FIG. 6 illustrates an example of a circuit 600 that supports time-basedaccess of a memory cell in accordance with various embodiments of thepresent disclosure. The circuit 600 may be configured to performtime-based access operations (e.g., read operations and writeoperations).

The circuit 600 may include a memory cell 602 coupled to a digit line604 and a plate line 606. The memory cell 602 may include a capacitor608 and a selection component 610. In some examples, a middle electrodemay be defined between the capacitor 608 and the selection component610. In some examples, the capacitor 608 may be a ferroelectriccapacitor. In some examples, the capacitor 608 may be a dielectriccapacitor. The selection component 610 may be coupled to an access line612 (e.g., word line) configured to activate the selection component 610based on instructions received from a memory controller. The memory cell602 may be an example of the memory cells described with reference toFIGS. 1-5. The digit line 604 may be an example of the digit lines 115described with reference to FIGS. 1-5. The plate line 606 may be anexample of the plate lines 210 described with reference to FIG. 2. Thecapacitor 608 may be an example of the capacitor 205 described withreference to FIG. 2. The selection component 610 may be an example ofthe selection component 220 described with reference to FIG. 2. Theaccess line 612 may be an example of the word line 110 described withreference to FIGS. 1 and 2.

At a first node 620, the memory cell 602 may couple to the digit line604. A charging component 622 may be coupled to the digit line 604 atthe first node 620. The charging component 622 may be configured tocharge the memory cell 602 and/or the digit line 604 to perform atime-based read operation. The charging component 622 may be coupled toa control line 624. The control line 624 may communicate instructionsfrom the memory controller 140 whether to charge the memory cell 602 ornot. The charging component 622 may be activated based on theinstructions from the memory controller 140. In some examples, thecharging component 622 is current generator. In some examples, thecharging component 622 is a cascode. In some examples, the chargingcomponent 622 may include one or more transistors.

An isolation component 626 may be coupled to the charging component 622at a second node 628 (Node). The isolation component 626 may beconfigured to selectively couple the second node 628 to a voltage source630 (Vpp) based on instructions received from the memory controller 140by a control line 632. In some examples, the isolation component 626 maybe an example of a transistor or other switching component.

A sense component 634 may be coupled to charging component 622. In someexamples, the sense component 634 may be coupled to the second node 628.As is more described with reference to FIG. 7, the sense component 634may be configured to detect when the digit line 604 at the first node620 charges to a predetermined voltage level. The sense component 634may be configured to detect when a voltage level at the second node 628satisfies a threshold voltage. In some examples, the sense component 634may be coupled to the voltage source 630. In some examples, the sensecomponent 634 may be an inverter. In some examples, the sense component634 may be components or circuitry configured to compare a voltage levelof the second node 628 to a voltage threshold (e.g., voltage threshold505). The sense component 634 may output a signal to a third node 636based on the voltage level of the second node satisfying the threshold.In some examples, the sense component 634 may sense a voltage level atthe second node 628. In some examples, the sense component 634 may becoupled to the digit line 604 at the first node 620.

A first latch 640 may be coupled to the sense component 634. In someexamples, the first latch 640 may be coupled to the third node 636. Thefirst latch 640 may be configured to output the value of the logic statestored on the selected memory cell 602. The first latch 640 may be usedas part of a time-based read operation where the value output by thefirst latch 640 may be based on a duration since the read operationbegan or since the memory cell 602 began to be charged.

The first latch 640 may be coupled to a first time-varying signal 642(the “F1 Signal”) by an access line. The first time-varying signal 642may be configured to indicate a logic state of the memory cell 602 basedon the duration between beginning to charge the memory cell 602 or thedigit line 604 and receiving a signal output from the sense component634. The signal being output from the sense component 634 may be basedon a voltage level satisfying a threshold. The first time-varying signal642 may be configured to define at least three logic states. In someexamples, the first time-varying signal 642 may be configured to defineat least two logic states. In some examples, the first time-varyingsignal 642 may be configured to define at least four logic states, orsome cases, more than four logic states.

In some examples, the memory controller 140 may apply the firsttime-varying signal 642 to the first latch 640 when the memory cell 602or digit line 604 begins to be charged by the charging component 622.The first time-varying signal 642 may be a predetermined time-varyingsignal based on the expected logic states of the memory cell 602. Thefirst time-varying signal 642 may vary in a predetermined manner over apredetermined time interval. In some examples, the first time-varyingsignal 642 may be received from a memory controller 140.

The first time-varying signal 642 may define a mapping between memorystates of the memory cell 602 and logic states of the memory cell 602.During a read operation, the charging component 622 may charge thememory cell 602. Based on the memory state of the memory cell 602 (e.g.,its polarization and/or dielectric charge), it will take a certain timeduration for a voltage associated with the memory cell 602 to satisfy avoltage threshold (e.g., voltage threshold 505). The first time-varyingsignal 642 may be configured to cycle through the possible logic statesof the memory cell 602. If the memory cell 602 is in a first memorystate A, the first time-varying signal 642 may be configured torepresent a first logic state associated with the first memory state Afor a subinterval of time. The subinterval of time being associated withan expected duration for the memory cell 602 to charge when the memorystate A. The first time-varying signal 642 may define a logic state foreach memory state of the memory cell 602 for a subinterval of the totaloverall interval. For example, the first time-varying signal 642 mayinclude a first subinterval defining a logic state associated with thememory state A. After the first subinterval, the first time-varyingsignal 642 may include a second subinterval defining a logic stateassociated with the memory state B. Such a pattern may continue untilthe memory states/logic states of the memory cell 602 are represented bythe first time-varying signal 642. In some examples, the subintervalsare substantially equal in duration. However, in other examples, thesubintervals may be different durations based on the expected chargingdurations of the memory cell 602.

In some examples, a second latch 644 may cooperate with the first latch640 to define the logic states of the memory cell 602. The second latch644 may be coupled to the third node 636 and to a second time-varyingsignal 646 (F2 Signal). The second time-varying signal 646 may cooperatewith the first time-varying signal 642 to define the logic states of thememory cell 602. Such an example, is described in more detail withreference to FIG. 8. In some examples, additional latches (not shown inFIG. 6) may be present. The additional latches may cooperate the withthe first latch and the second latch to define more logic states of thememory cell by dividing the reading time duration in finer granularitytime subintervals.

A controller 660 may be coupled to the first latch 640 by a data line648 and to the second latch 644 by a data line 650. The controller 660may be configured to identify the logic state of the memory cell 602based on the value of the first time-varying signal 642 received fromthe first latch 640. In some examples, identifying the logic state ofthe memory cell 602 may be based on both the first time-varying signal642 and the second time-varying signal 646 received from the secondlatch 644. The controller 660 may also be configured to execute a writeback operation as part of the read operation. In some examples, thecontroller 660 may be configured to identify a first bit of the logicstate prior to identifying a second bit of the logic state. For example,if the memory cell 602 is capable of storing four logic states (00, 01,10, 11), the controller 660 may be configured to identify whether themost-significant bit of a memory identifier is a logical ‘1’ or alogical ‘0’ prior to identifying the value of the other bit.

The controller 660 may also operate the switching components 662, 664,666 to perform a write back portion of a read operation. In someexamples, the controller 660 may be configured to perform writeoperations as part a normal write operation. The controller 660 may becoupled to the switching components 662, 664 by a first control line668. The controller 660 may be coupled to the switching component 666 bya second control line 670. In some examples, any number of control linesmay be used by the controller 660 to operate the switching components662, 664, 666.

The switching component 662 may be coupled to a voltage source 672 (Vo).The switching component 662 may be configured to bias the plate line 606high (e.g., to the voltage source 672) during a write operation or awrite back operation. The switching component 662 may be a transistor orother type of switching component.

The switching component 664 may be coupled to a ground 674. Theswitching component 664 may be configured to bias the plate line 606 low(e.g., to ground) during a write operation or a write back operation. Insome examples, the ground 674 may be a ground or a virtual ground thatis a voltage source at a Vss.

In the instances when the same control line is used to control both theswitching component 662 and the switching component 664 (e.g., firstcontrol line 668), the switching component 662 may be configured to beactivated when the switching component 664 is deactivated. As such, theswitching component 662 may be configured to be activated based on a lowsignal, while the switching component 664 may be configured to beactivated based on a high signal, or vice versa.

The switching component 666 may be coupled to the ground 674. Theswitching component 664 may be configured to bias the digit line 604 low(e.g., to ground or virtual ground) during a write operation or a writeback operation. In some examples, the controller may be configured theword line 612 during a write or write-back operation. Such control ofthe word line 612 may be used when the word line 612 is deactivatedafter the dielectric charging of the memory cell 602.

In some examples, the controller 660 may be coupled to another switchingcomponent to bias the digit line 604 high during a write or write backoperation. In some examples, the charging component 622 may be operatedto bias the digit line 604 high during a write or write back operation.

In some instances, the controller 660 may be an example of the memorycontroller 140. In some instances, the controller 660 may be a dedicatedcomponent, a dedicated circuit, or dedicated logic configured to performthe functions described herein. In some instances, the controller 660may be coupled to the memory controller 140 and may be configured tocooperate with the memory controller 140 to perform the variousfunctions described herein. For example, the controller 660 may performsome portions of the functions described herein and the memorycontroller 140 may perform the other portions of the functions describedherein, in some examples.

FIG. 7 illustrates an example of a timing diagram 700 that supportstime-based access of a memory cell in accordance with variousembodiments of the present disclosure. The timing diagram 700illustrates a digit line voltage signal 705 of the digit line 604 at thefirst node 620 and a node voltage signal 710 at the second node 628 ofthe circuit 600. The digit line voltage signal 705 and the node voltagesignal 710 may represent voltages during a read operation of a memorycell 602. More specifically, the signals 705, 710 may represent voltagesduring a sense portion of the read operation.

A read operation performed on the memory cell 602 may include apreconditioning portion, a sense portion, a write back operation, and aprecharge portion. At time t0, a memory controller 140 may initiate asense portion of the read operation. To develop the signal from thememory cell 602, the memory controller 140 may activate the chargingcomponent 622 to charge the memory cell 602 or the digit line 604 to thepredetermined voltage level 715 as represented by Vd1 in FIG. 7. Thevoltage level of the memory cell 602 rises from a starting voltage level720 (represented by Vst in FIG. 7) to the predetermined voltage level715 (Vd1).

The voltage level of the second node 628, as represented by the nodevoltage signal 710, also rises from a starting voltage level based onthe memory cell 602 being charged. The starting voltage level of thesecond node 628 may be based on the starting voltage level of the digitline 604 and/or memory cell 602. In some examples, the starting voltagelevel of the second node 628 may be the same as the starting voltagelevel of the digit line 604 and/or memory cell 602. In some examples,the starting voltage level of the second node 628 may be different fromthe starting voltage level of the digit line 604 and/or memory cell 602.In some examples, the starting voltage varies based on a memory state ofthe memory cell 602 being charged.

A voltage threshold 725 may be defined for the voltage level of thesecond node 628 (represented by node voltage signal 710). The voltagethreshold 725 may be associated with when the voltage level of the digitline 604 and/or memory cell 602 reaches the predetermined voltage level715. The voltage threshold 725 may be selected based on an identifiedrelationship between the voltage level of the second node 628 and avoltage level of the first node 620. In some examples, the voltagethreshold 725 may be an example of the voltage threshold 505 describedwith reference to FIG. 5.

At time t1, the voltage level at the second node 628 may satisfy thevoltage threshold 725. The circuit 600 may make this determination usingthe sense component 634 in some examples. In some instances, the sensecomponent 634 may compare the voltage level detected at the second node628 to a reference voltage to identify with the voltage threshold 725 issatisfied. At time t1, the sense component 634 may output a signal tothe latch 640 based on the voltage level at the second node 628satisfying the voltage threshold 725. The voltage threshold 725 may bemodified or altered based on circuit operation or changes in the accessoperations.

A duration 730 may be defined between the beginning of charging thedigit line 604 and/or memory cell 602 at time t0 and when the voltagelevel satisfies the voltage threshold 725 at time t1. The duration 730may correspond to one of the durations described with reference to FIG.5. The duration 730 may vary based on the memory state of the memorycell 602 at time t0 when the charging begins. The starting voltagelevels of both the digit line 604 and the second node 628 may also varybased on the memory state of the memory cell 602 at time t0 when thecharging begins. For example, the starting voltage levels for memorystate A (described with reference to FIGS. 3 and 5) may be higher thanmemory state D (described with reference to FIGS. 3 and 5). In someexamples, the signals 705, 710 may vary based on the memory state of thememory cell 602 at time t0 when the charging begins.

In some examples, the sense component 634 of the circuit 600 may becoupled to the digit line 604 at the first node 620. In those examples,the voltage threshold 725 may be set to be at or around thepredetermined voltage level 715 that the digit line 604 and/or thememory cell 602 are being charged. It should be appreciated that theelements of the sense component 634 may be modified when the sensecomponent 634 is coupled to digit line 604 to perform the functionsdescribed herein.

In some examples, the digit line 604 may be biased prior to starting theread operation. Biasing the digit line 604 may reduce disturbances ofthe logic state of unselected memory cells also coupled to the digitline 604. Biasing the digit line 604 before performing the readoperation, in some cases, may not alter the durations taken to chargethe digit line 604 during a sense portion of a read operation.

In some instances, a time-based read operation may be performed on amemory cell 602 without a latch and/or time-varying signals. In someinstances, the controller 660 or memory controller 140 may determine aduration between beginning to charge the memory cell 602 and when thevoltage threshold 725 is satisfied. The controller 660 or memorycontroller 140 may compare the duration to values of a look-up table.The look-up table may be configured to map durations to a particularlogic state. In some examples, a timer may be initiated when the memorycell 602 begins to be charged. The duration of the sense portion of theread operation may be based on a value of the timer when the voltagethreshold is satisfied.

A write back portion of the read operation may begin after the logicstate of the memory cell 602 is identified. The controller 660 mayidentify the logic state of the memory cell 602 about or after time t1.The controller 660 may then determine what memory state should bewritten to the memory cell based on the identified logic state. In someexamples, the memory state to be written back is the same memory statethat was identified by the controller 660.

During a sense portion of a time-based read operation, the digit line604 may be charged or biased to a high voltage. To write memory statesto the memory cell 602, the memory cell 602 may be biased by the digitline 604 and the plate line 606. To write some memory states to thememory cell 602, the digit line 604 may be high and the plate line 606may be low. To write other memory states to the memory cell 602, thedigit line 604 may be low and the plate line 606 may be high.

As such, to write some memory states to the memory cell 602, thecontroller 660 may activate the switching component 664 to couple theplate line 606 to ground 674. Because the digit line 604 may already behigh due to the charging during the sense portion of the read operation,the memory cell 602 may be biased to write back certain memory state tothe memory cell 602.

In some instances, the digit line 604 may not be high at the beginningof a write operation or a write back portion of a read operation. Insuch instances, the controller 660 may activate one or more switchingcomponents (not shown) to couple the digit line 604 to a voltage sourceand the switching component 664 may be activated to couple the plateline 606 to ground. For example, during a normal write operation, thedigit line 604 may be at a low value at the beginning of the writeoperation. In another example, the digit line 604 may be coupled toground after the sense component detects that the threshold has beensatisfied. Switching the memory cell 602 off during portions of the readoperation may reduce the stress on the memory cell 602. In suchexamples, the digit line 604 may be coupled to one or more switchingcomponents that selectively couple the digit line 604 to ground.

To write other memory states to the memory cell 602, the controller 660may activate the switching component 662 to couple the plate line 606 toa voltage source and may activate the switching component 666 to couplethe digit line 604 to ground 674. In some cases, prior to activating theswitching component 666, the charging component 622 may be deactivated.

In some examples, the controller 660 may deactivate the selectioncomponent 610 during a write operation or a write back operation. Insuch examples, the selection component 610 may be deactivated while theplate line 606 or the digit line 604 is high. Deactivating the selectioncomponent 610 may cause the middle electrode of the memory cell 602 tostore a dielectric charge. In some examples, deactivating the selectioncomponent 610 may cause the capacitor 608 to store a dielectric charge.In some examples, a ferroelectric memory cell may be configured to storeboth a polarization state and a dielectric charge state by notdischarging the middle electrode after a write operation or a write backoperation.

In some examples, the memory cell 602 may be configured to store aplurality of memory states. As such, the controller 660 may be coupledto a plurality of switching components coupled to a plurality of voltagesources. Various combinations of these voltage sources may be used toobtain the proper biasing for the memory cell 602. For example, a memorycell 602 may be written with a memory state by coupling the digit line604 to a first voltage and coupling the plate line 606 to a secondvoltage different from the voltage. The first and second voltage may beany voltage. The circuit 600 may include any number of control lines andswitching components to properly write various memory states to thememory cell 602.

FIG. 8 illustrates an example of a timing diagram 800 that supportstime-based access of a memory cell in accordance with variousembodiments of the present disclosure. The timing diagram 800illustrates an examples of time-varying signals input into at least onelatch (e.g., latch 640). The timing diagram 800 includes a firsttime-varying signal 805 and a second time-varying signal 810. In someexamples, the time-varying signals 805, 810 may be input into a singlelatch (e.g., first latch 640). In some examples, the time-varyingsignals 805, 810 may be input into two latches (e.g., first latch 640and second latch 644). The first time-varying signal 805 may be anexample of the first time-varying signal 642 described with reference toFIG. 6. The second time-varying signal 810 may be an example of thesecond time-varying signal 646 described with reference to FIG. 6. Insome examples, the amplitude of the signals 805, 810 may be varied overtime. In other examples, other characteristics of the signals 805, 810may be varied over time.

The first and second time-varying signals 805, 810 may be configured todefine logic states stored on a memory cell 602. The first and secondtime-varying signals 805, 810 may be configured to represent logical‘1s’ and logical ‘0s’ based on high and low voltage values. For example,a high voltage value of the time-varying signals 805, 810 may representa logical ‘1’ and a low voltage value may represent a logical ‘0.’

In a time-based read operation, the duration between the beginning ofcharging the memory cell 602 to a predetermined voltage level (e.g.,voltage level 715) and satisfying a voltage threshold (e.g., voltagethreshold 725) may be used to activate one or more latches (e.g.,latches 640, 644). The value of the time-varying signals 805, 810 at thetime the latches are activated may be used to identify the logic stateof the memory cell 602. For example, if the charging of the memory cell602 begins at time t0 and the voltage threshold is satisfied at time t1,the value of the first time-varying signal 805 may indicate that thefirst bit of a logic state of the memory cell 602 is a logical ‘0’ andthe second bit is a logical ‘0.’

The first and second time-varying signals 805, 810 may cooperate to mapa logic state of the memory cell 602 to an associated memory state ofthe memory cell 602 based on the duration to charge the memory cell 602during a read operation. Such time-based read operations may be used todistinguish between memory states not previously distinguishable inother memory cells. For example, a time-based read operation may be ableto distinguish between a first memory state defined by zero polarizationand a first level of dielectric charge and a second memory state definedby a first polarization and the first level of dielectric charge. Insome examples, time-based read operations may be configured todistinguish between different levels of dielectric charge alone ordifferent levels of polarization alone, or change in both.

The first and second time-varying signals 805, 810 may be based onexpected durations of charging associated with different memory statesof a memory cell 602. As used in FIG. 8, the time t1 may represent thetime at which the voltage satisfies the voltage threshold during a readoperation when the memory cell 602 stores the memory state A. A duration815 defined between time t0 and time t1 may correspond to duration 530described with reference to FIG. 5. As used in FIG. 8, the time t2 mayrepresent the time at which the voltage satisfies the voltage thresholdduring a read operation when the memory cell 602 stores the memory stateB. A duration 820 defined between time t0 and time t2 may correspond toduration 540 described with reference to FIG. 5. As used in FIG. 8, thetime t3 may represent the time at which the voltage satisfies thevoltage threshold during a read operation when the memory cell 602stores the memory state C. A duration 825 defined between time t0 andtime t3 may correspond to duration 550 described with reference to FIG.5. As used in FIG. 8, the time t4 may represent the time at which thevoltage satisfies the voltage threshold during a read operation when thememory cell 602 stores the memory state D. A duration 830 definedbetween time t0 and time t4 may correspond to duration 555 describedwith reference to FIG. 5.

The first and second time-varying signals 805, 810 may be configured toextend for an overall interval 835. The overall interval 835 may includea number of subintervals. Each subinterval may define a unique logicstate of the memory cell 602. For instance, in the examples where thememory cell 602 is configured to store four memory states, the first andsecond time-varying signals 805, 810 may define four subintervals. Eachsubinterval may be associated with a separate memory state of the memorycell 602. Each subinterval may be associated with an expected durationfor charging for a separate memory state of the memory cell 602.

A subinterval may represent a time period during which a single logicstate of the memory cell 602 is represented by one or more time-varyingsignals. In the illustrative example, two time-varying signals are usedto represent the possible logical states of a memory cell 602. However,in other examples, other number of time-varying signals may be used torepresent the possible logical states of a memory cell 602 (e.g., onetime-varying signal, three time-varying signals, etc.). The first andsecond time-varying signals 805, 810 may include a first subinterval840, a second subinterval 845, a third subinterval 850, and fourthsubinterval 855. In the representative example of FIG. 8, the firstsubinterval 840 may represent a logical ‘00,’ the second subinterval 845may represent a logical ‘01,’ the third subinterval 850 may represent alogical ‘10,’ and a fourth subinterval 855 may represent a logical ‘11.’In some examples, the first time-varying signal 805 may represent amost-significant bit of a logic state identifier and the secondtime-varying signal 810 may represent a least-significant bit of a logicstate identifier. In some examples, a single time-varying signal mayrepresent more two or more bits of a logic state identifier.

Each subinterval may be separated by a transition. A transition mayrefer to a change in the voltage level of one of the time-varyingsignals or both of the time-varying signals. The change in the voltagelevel may represent a change in the logic state be represented by theone or more time-varying signals. The first subinterval may extendbetween an initial transition at time t0 (e.g., the beginning ofapplying the time-varying signal) and a first transition 860. The secondsubinterval 845 may extend between the first transition 860 and a secondtransition 865. The third subinterval 850 may extend between the secondtransition 865 and a third transition 870. The fourth subinterval 855may extend between the third transition 870 and a fourth transition 875or an ending transition.

At the first transition 860, the first time-varying signal 805 may notalter its voltage value and the second time-varying signal 810 may alterits voltage value from a low voltage value to a high voltage value. Atthe second transition 865, the first time-varying signal 805 may alterits voltage value from low to high and the second time-varying signal810 may alter its voltage value from high to low. At the thirdtransition 870, the first time-varying signal 805 may not alter itsvoltage value and the second time-varying signal 810 may alter itsvoltage value from low to high. At the fourth transition 875, the firsttime-varying signal 805 may alter its voltage value from high to low andthe second time-varying signal 810 may alter its voltage value from highto low.

In some examples, the subintervals may span equal lengths of time.However, in other examples, the subintervals may span different lengthsof the time. The transitions of the time-varying signals between logicalstates may be positioned to distinguish between memory states of thememory cell 602. Because a read operation may not produce equally spacedapart durations for satisfying voltage thresholds, similarly thethresholds between logical states may not be equally spaced.

The first and second time-varying signals 805, 810 may be used inconjunction with the memory states A-D shown and described withreference to FIGS. 3 and 5. In such an example, the subinterval 840 maybe associated with the memory state A (e.g., memory state 340 (StateA)), the subinterval 845 may be associated with the memory state B(e.g., memory state 305 (State B)), the subinterval 850 may beassociated with the memory state C (e.g., memory state 310 (State C)),and the subinterval 855 may be associated with the memory state D (e.g.,memory state 345 (State D)). As such, in this example, memory state Amay be mapped to a logical ‘00,’ memory state B may be mapped to alogical ‘01,’ memory state C may be mapped to a logical ‘10,’ and memorystate D may be mapped to a logical ‘11.’ In some examples, thetime-varying signals may be configured to map the memory states to anylogic state. The mapping shown FIG. 8 is provided for illustrativepurposes only.

In some examples, the time-varying signal(s) may be configured such thata first bit of a logic state identifier may be identified after a firstduration shorter than a second duration to determine a second bit of thelogic state identifier. For example, at transition 865 the controller660 may be able to determine whether one of the bits is a logical ‘1’ ora logical ‘0.’ If the voltage threshold has not been satisfied by thetransition 865, then the controller 660 may determine that the first bitis a logical ‘1.’ This type of determination may be accomplished throughan inference. In some examples, if the voltage threshold has not beensatisfied by the transition 870, the controller 660 may determine thatlogical state identifier is a logical ‘11.’ Such a determination may bedone by inference because during the time-based sense there is no longerany ability to determine that any of the other three logic statesrepresented in the timing diagram 800 is stored by the memory cell 602.In some examples, the overall interval 835 of the time-varying signalsmay terminate at transition 870. As such, the time-varying signals mayinclude three subintervals 840, 845, 850 and not include subinterval855. In other examples, however, the time-varying signals extend to thetransition 875 to identify whether an error has occurred during a readoperation. If the voltage threshold is never satisfied during theinterval 835, then the controller 660 may determine that an error in theread operation occurred.

FIG. 9 illustrates an example of timing diagrams 900 that supporttime-based access of a memory cell in accordance with variousembodiments of the present disclosure. The timing diagrams 900 representexpected durations for charging for various memory state of the memorycell 602. The timing diagram 900-a may represent the expected durationsfor charging when the charging component 622 applies a constant currentto the digit line 604 as part of the read operation. The timing diagram900-b may represent the expected durations for charging when thecharging component 622 applies a time-varying current to the digit line604 as part of the read operation.

The duration of time taken to charge the digit line 604 and/or thememory cell 602 may be based on the characteristics of the components ofthe memory device. Because characteristics of components (e.g.,capacitance) of the memory device are fixed, the duration to charge thememory cell 602 may be based on the memory state of the memory cell 602and how that memory state interacts with the other fixed characteristicsof the components of the circuit.

For example, if the capacitances associated with the circuit are fixedand the charging component applies a constant current or a constantpower supply during charging, expected values of the durationsassociated with each memory state may be determined. At time t0, thecharging of the memory cell 602 as part of a sense portion of a readoperation begins. At time t1, a memory cell 602 having the memory stateA (e.g., memory state 340 (State A)) satisfies the voltage threshold. Insome examples, a duration 905 defined between time t0 and time t1 isabout zero nanoseconds. In some instances, the duration 905 may be morethan zero nanoseconds such as 0.2 nanoseconds, 0.4 nanoseconds, 0.6nanoseconds, 0.8 nanoseconds, 1.0 nanoseconds, etc. Frequently, thevoltage threshold and the predetermined voltage for charging may be setbased on one of the memory states of the memory cell 602. As such, oneof the memory states of the memory cell 602 may satisfy the voltagethreshold quickly, and sometimes instantaneously after charging begins.Time t1 is shown as different from time t0 for illustrative purposesonly. In some examples, time t1 occurs at or directly after time t0.

At time t2, a memory cell 602 having the memory state B (e.g., memorystate 305 (State B)) satisfies the voltage threshold. In some examples,a duration 910 defined between time t0 and time t2 is about tennanoseconds. In some instances, the duration 910 may range between 7nanoseconds and 13 nanoseconds, 7.5 nanoseconds and 12.5 nanoseconds, 8nanoseconds and 12 nanoseconds, 8.5 nanoseconds and 11.5 nanoseconds,9.0 nanoseconds and 11 nanoseconds, or 9.5 nanoseconds and 10.5nanoseconds.

At time t3, a memory cell 602 having the memory state C (e.g., memorystate 310 (State C)) satisfies the voltage threshold. In some examples,a duration 615 defined between time t0 and time t3 is about forty-twonanoseconds. In some instances, the duration 915 may range between 35nanoseconds and 49 nanoseconds, 36 nanoseconds and 48 nanoseconds, 37nanoseconds and 47 nanoseconds, 38 nanoseconds and 46 nanoseconds, 39nanoseconds and 45 nanoseconds, 40 nanoseconds and 44 nanoseconds, 41.0nanoseconds and 43 nanoseconds or 41.5 nanoseconds and 42.5 nanoseconds.

At time t4, a memory cell 602 having the memory state D (e.g., memorystate 345 (State D)) satisfies the voltage threshold. In some examples,a duration 920 defined between time t0 and time t4 is about fifty-twonanoseconds. In some instances, the duration 920 may range between 45nanoseconds and 59 nanoseconds, 46 nanoseconds and 58 nanoseconds, 47nanoseconds and 57 nanoseconds, 48 nanoseconds and 56 nanoseconds, 49nanoseconds and 55 nanoseconds, 50 nanoseconds and 54 nanoseconds, 51.0nanoseconds and 53 nanoseconds, or 51.5 nanoseconds and 52.5nanoseconds.

The relationships between the durations 910, 915, 920 may be based onthe capacitances of the circuit. Because the design of the circuit andthe characteristics of those circuit components is relatively constant,applying a constant current may yield predictable durations for chargingthe memory cell 602 based on the memory states. The durations and rangevalues described above may be based on a value of the current used tocharge the memory cell 602 and/or the digit line 604. Thus, in somecases, a high higher may result in less time to satisfy the threshold(e.g., twice the current may result in half the time to satisfy thethreshold).

As should be appreciated, the durations of timing diagram 900-a may makedistinguishing between some memory states to be more difficult thandistinguishing between other memory states. A first sense window 925based on time between memory state A (time t1) and memory state B (timet2) may be about ten nanoseconds, in this example. A second sense window930 based on time between memory state B (time t2) and memory state C(time t3) may be about thirty-two nanoseconds. A third sense window 935based on time between memory state C (time t3) and memory state D (timet4) may be about ten nanoseconds.

Because of the relative durations of the sense windows 925, 930, 935, itmay be more difficult or less difficult to distinguish between memorystates in a time-based read operation. For example, because the firstsense window 925 is about ten nanoseconds and the second sense window930 is about three times the size of the first sense window, it may beeasier to distinguish between memory state B and memory state C than itis to distinguish between memory state A and memory state B.

In some examples, the current or power source applied to the memory cell602 during a sense portion of a read operation may be varied over time.Such a time-varying current may be configured to distribute thedurations for charging in a predetermined manner. For example, atime-varying current applied by the charging component 622 may beconfigured to provide equally sized sense windows based on time. In someexamples, the amplitude of the current may be varied over time. In otherexamples, other characteristics of the current may be varied over time.

The timing diagram 900-b illustrates durations and sense windowsassociated with a sense portion of a time-based read operation. In theread operation, a time-varying current is applied. The time-varyingcurrent is configured to change the charge times associated differentmemory states. For example, a duration 950 associated with memory stateB may be longer than a duration 910. In another example, a duration 955associated with memory state C may be shorter than the duration 915. Insome examples, the duration 960 associated with memory state D may bedifferent than the duration 920. In some examples, the time-varyingcurrent may be configured to make the duration 960 shorter than theduration 920 and thereby reduce the overall time taken during a senseportion of a read operation. In some examples, the time-varying currentmay be configured to provide predetermined sense windows, and as suchthe duration 960 may be longer than the duration 920.

The sense windows 965, 970, 975 in the timing diagram 900-b may be aboutequal in length of time. The length of time of the sense windows 965,970, 975 may be based on the configuration of the time-varying currentapplied while charging the memory cell 602. In some examples, otherconfigurations of durations and sense windows may be based on differentcurrent profiles of the time-varying current applied while charging thememory cell 602.

FIG. 10 shows a block diagram 1000 of a memory array 1005 that supportstime-based access of a memory cell in accordance with variousembodiments of the present disclosure. Memory array 1005 may be referredto as an electronic memory apparatus, and may be an example of acomponent of a memory controller 140 as described with reference to FIG.1.

Memory array 1005 may include one or more memory cells 1010, a memorycontroller 1015, a word line 1020, a plate line 1025, a referencecomponent 1030, a sense component 1035, a digit line 1040, and a latch1045. These components may be in electronic communication with eachother and may perform one or more of the functions described herein. Insome cases, memory controller 1015 may include biasing component 1050and timing component 1055. In some examples, the memory controller 1015may be an example of a memory controller 140 as described with referenceto FIG. 1. In some examples, the memory controller 1015 may be anexample of a controller 660 as described with reference to FIG. 6. Insome examples, the memory controller 1015 may be an example of both thememory controller 140 and the controller 660.

Memory controller 1015 may be in electronic communication with word line1020, digit line 1040, sense component 1035, and plate line 1025, whichmay be examples of word line 110, digit line 115, sense component 125,and plate line 210 described with reference to FIGS. 1, and 2. Memoryarray 1005 may also include reference component 1030 and latch 1045. Thecomponents of memory array 1005 may be in electronic communication witheach other and may perform portions of the functions described withreference to FIGS. 1 through 9. In some cases, reference component 1030,sense component 1035, and latch 1045 may be components of memorycontroller 1015.

In some examples, digit line 1040 is in electronic communication withsense component 1035 and a ferroelectric capacitor of ferroelectricmemory cells 1010. A ferroelectric memory cell 1010 may be writable witha logic state (e.g., a first or second logic state). Word line 1020 maybe in electronic communication with memory controller 1015 and aselection component of ferroelectric memory cell 1010. Plate line 1025may be in electronic communication with memory controller 1015 and aplate of the ferroelectric capacitor of ferroelectric memory cell 1010.Sense component 1035 may be in electronic communication with memorycontroller 1015, digit line 1040, latch 1045, and reference line 1060.Reference component 1030 may be in electronic communication with memorycontroller 1015 and reference line 1060. Sense control line 1065 may bein electronic communication with sense component 1035 and memorycontroller 1015. These components may also be in electroniccommunication with other components, both inside and outside of memoryarray 1005, in addition to components not listed above, via othercomponents, connections, or busses.

Memory controller 1015 may be configured to activate word line 1020,plate line 1025, or digit line 1040 by applying voltages to thosevarious nodes. For example, biasing component 1050 may be configured toapply a voltage to operate memory cell 1010 to read or write memory cell1010 as described above. In some cases, memory controller 1015 mayinclude a row decoder, column decoder, or both, as described withreference to FIG. 1. This may enable memory controller 1015 to accessone or more memory cells 105. Biasing component 1050 may also providevoltage potentials to reference component 1030 in order to generate areference signal for sense component 1035. Additionally, biasingcomponent 1050 may provide voltage potentials for the operation of sensecomponent 1035.

In some cases, memory controller 1015 may perform its operations usingtiming component 1055. For example, timing component 1055 may controlthe timing of the various word line selections or plate biasing,including timing for switching and voltage application to perform thememory functions, such as reading and writing, discussed herein. In somecases, timing component 1055 may control the operations of biasingcomponent 1050. In some examples, the timing component 1055 maycooperate to generate the F1 signal and/or the F2 signal.

Reference component 1030 may include various components to generate areference signal for sense component 1035. Reference component 1030 mayinclude circuitry configured to produce a reference signal. In somecases, reference component 1030 may be implemented using otherferroelectric memory cells 105. Sense component 1035 may compare asignal from memory cell 1010 (through digit line 1040) with a referencesignal from reference component 1030. Upon determining the logic state,the sense component may then store the output in latch 1045, where itmay be used in accordance with the operations of an electronic devicethat memory array 1005 is a part. Sense component 1035 may include asense amplifier in electronic communication with the latch and theferroelectric memory cell.

Memory controller 1015 may be an example of portions of the memorycontroller 1215 described with reference to FIG. 12. Memory controller1015 and/or at least some of its various sub-components may beimplemented in hardware, software executed by a processor, firmware, orany combination thereof. If implemented in software executed by aprocessor, the functions of the memory controller 1015 and/or at leastsome of its various sub-components may be executed by a general-purposeprocessor, a digital signal processor (DSP), an application-specificintegrated circuit (ASIC), an field-programmable gate array (FPGA) orother programmable logic device, discrete gate or transistor logic,discrete hardware components, or any combination thereof designed toperform the functions described in the present disclosure. The memorycontroller 1015 and/or at least some of its various sub-components maybe physically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations by one or more physical devices. In some examples, memorycontroller 1015 and/or at least some of its various sub-components maybe a separate and distinct component in accordance with variousembodiments of the present disclosure. In other examples, memorycontroller 1015 and/or at least some of its various sub-components maybe combined with one or more other hardware components, including butnot limited to an I/O component, a transceiver, a network server,another computing device, one or more other components described in thepresent disclosure, or a combination thereof in accordance with variousembodiments of the present disclosure.

Memory controller 1015 may charge a digit line coupled to a memory cellto a first voltage level, determine a duration for the digit line tocharge to the first voltage level, and identify a logic state of thememory cell based on the duration for the digit line to reach the firstvoltage level. The memory controller 1015 may also force a current intoa ferroelectric memory cell coupled to a digit line, the ferroelectricmemory cell configured to store at least three logic states, sense avoltage at a node different from the digit line, the voltage based on afirst voltage level of the digit line, and identify a logic state of theferroelectric memory cell from the at least three logic states based onthe voltage satisfying a voltage threshold. The memory controller 1015may also apply a time-varying signal to a latch after initiating a readoperation on a memory cell, activate the latch based on a digit linethat is coupled to the memory cell charging to a first voltage level aspart of the read operation, and identify a logic state of the memorycell based on a value of the time-varying signal present at the latchwhen the latch is activated. The memory controller 1015 may also sense afirst state of a ferroelectric capacitor in a ferroelectric memory cell,sense a second state of the ferroelectric capacitor different from thefirst state, and identify a logic state of the ferroelectric memory cellfrom at least three logic states based on the first state and the secondstate. The memory controller 1015 may also activate a selectioncomponent of a ferroelectric memory cell, modify a first state of aferroelectric capacitor of the ferroelectric memory cell based on avoltage being applied to the ferroelectric memory cell while theselection component is activated, deactivate the selection component,and modify a second state of the ferroelectric capacitor based on theselection component being deactivated while the voltage is applied tothe ferroelectric memory cell.

In some cases, the memory array 1005 may include various means foroperating the memory array 1005. For example, the memory array 1005and/or the memory controller 1015 may include means for performing thefunctions described above with reference to FIG. 13.

The memory array 1005 may include means for charging a digit linecoupled to a memory cell to a first voltage level, means for determininga duration for the digit line to charge to the first voltage level, andmeans for identifying a logic state of the memory cell based at least inpart on the duration for the digit line to reach the first voltagelevel.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for varying anamplitude of a current applied to the digit line over time, wherein theduration is based at least in part on the time-varying current. Someexamples of the memory array 1005 described above may further includeprocesses, features, means, or instructions for selecting the memorycell from a plurality of memory cells based at least in part on a readoperation being initiated.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for initiating atimer based at least in part on performing a read operation on thememory cell, wherein the duration may be determined based at least inpart on the timer. In some examples of the memory array 1005 describedabove, the duration may be determined based at least in part on anamount of time that elapses between initiating a timer and the digitline charging to the first voltage level.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for determining thata second voltage level at a node different from the digit line satisfiesa voltage threshold, wherein the duration may be based at least in parton the second voltage level satisfying the voltage threshold.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for sensing, by asense component, the second voltage level at the node, wherein the logicstate may be identified based at least in part on the second voltagelevel satisfying the threshold. In some examples of the memory array1005 described above, the voltage threshold may be less than a biasingvoltage used to generate a stable state of the memory cell.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for identifying avalue of a time-varying signal based at least in part on the duration,wherein the logic state may be based at least in part on the value ofthe time-varying signal. Some examples of the memory array 1005described above may further include processes, features, means, orinstructions for biasing the digit line prior to charging the digit linewith the first voltage level.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for identifying afirst bit of the logic state after a second duration less than theduration. Some examples of the memory array 1005 described above mayfurther include processes, features, means, or instructions foridentifying a second bit of the logic state after the duration. In someexamples of the memory array 1005 described above, the first voltagelevel to which the digit line may be charged may be a predeterminedvoltage level based at least in part on at least on one of a pluralityof possible charge states of the memory cell.

In some examples of the memory array 1005 described above, the digitline may be charged by a cascode coupled to the digit line and to asense component. In some examples of the memory array 1005 describedabove, the duration may be based at least in part on a stable state of acapacitor of the memory cell and a volatile state of the capacitor ofthe memory cell.

In some examples of the memory array 1005 described above, the memorycell includes a ferroelectric capacitor. In some examples of the memoryarray 1005 described above, the memory cell includes a dielectriccapacitor. In some examples of the memory array 1005 described above,the memory cell may be configured to store at least three logic states.In some examples of the memory array 1005 described above, the memorycell may be configured to store two logic states.

The memory array 1005 may include means for applying a current to aferroelectric memory cell coupled to a digit line, the ferroelectricmemory cell configured to store at least three logic states, means forsensing a voltage at a node different from the digit line, the voltagebased at least in part on a first voltage level of the digit line, andmeans for identifying a logic state of the ferroelectric memory cellfrom the at least three logic states based at least in part on thevoltage satisfying a voltage threshold.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for identifying aduration for a second voltage level present on the node to satisfy thevoltage threshold, wherein the logic state may be identified based atleast in part on the duration.

In some examples of the memory array 1005 described above, the durationmay be based at least in part on a total charge stored on aferroelectric capacitor of the ferroelectric memory cell. In someexamples of the memory array 1005 described above, the total chargecomprises a volatile charge of the ferroelectric capacitor and a stablecharge of the ferroelectric capacitor.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for identifying thelogic state of the ferroelectric memory cell may be based at least inpart on a polarization state of a ferroelectric capacitor of theferroelectric memory cell and a charge state of the ferroelectriccapacitor of the ferroelectric memory cell.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for outputting, by asense component, a signal based at least in part on the voltagesatisfying the voltage threshold. Some examples of the memory array 1005described above may further include processes, features, means, orinstructions for activating a first latch based at least in part on thevoltage satisfying the voltage threshold. Some examples of the memoryarray 1005 described above may further include processes, features,means, or instructions for activating a second latch different from thefirst latch based at least in part on the voltage satisfying the voltagethreshold.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for applying a firsttime-varying signal to the first latch. Some examples of the memoryarray 1005 described above may further include processes, features,means, or instructions for applying a second time-varying signal to thesecond latch different from the first time-varying signal, wherein thelogic state of the ferroelectric memory cell may be based at least inpart on values of the first time-varying signal and the secondtime-varying signal when the first latch and the second latch may beactivated.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for activating aselection component of the ferroelectric memory cell, wherein thecurrent may be forced based at least in part on the selection componentbeing activated.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for activating aselection component of the ferroelectric memory cell while a plate lineand the digit line coupled to the ferroelectric memory cell may begrounded or virtually grounded.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for performing awrite-back operation on the ferroelectric memory cell based at least inpart on the identified logic state of the ferroelectric memory cell. Insome examples of the memory array 1005 described above, the current maybe forced based at least in part on performing a read operation on theferroelectric memory cell.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for charging, by acurrent generator, the digit line to the first voltage level based atleast in part on applying the current. In some examples of the memoryarray 1005 described above, the current may be forced by a currentgenerator coupled to the digit line and the node.

The memory array 1005 may include means for applying a time-varyingsignal to a latch after initiating a read operation on a memory cell,means for activating the latch based at least in part on a digit linethat is coupled to the memory cell charging to a first voltage level aspart of the read operation, and means for identifying a logic state ofthe memory cell based at least in part on a value of the time-varyingsignal present at the latch when the latch is activated.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for isolating thememory cell from the latch after the digit line charges to the firstvoltage as part of the read operation.

In some examples of the memory array 1005 described above, the memorycell may be configured to store at least three logic states. In someexamples of the memory array 1005 described above, the identified logicstate of the memory cell may be selected from the at least three logicstates.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for applying asecond time-varying signal to a second latch based at least in part onperforming the read operation on the memory cell, the secondtime-varying signal different from the time-varying signal, the secondlatch different from the latch.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for activating thesecond latch based at least in part on the digit line that may becoupled to the memory cell charging to the first voltage level, whereinidentifying the logic state of the memory cell may be based at least inpart on the time-varying signal present at the latch and the secondtime-varying signal present at the second latch when the latch and thesecond latch may be activated.

In some examples of the memory array 1005 described above, aconfiguration of the second time-varying signal may be based at least inpart on a configuration of the time-varying signal, wherein thetime-varying signal and the second time-varying signal cooperate todefine at least three logic states. Some examples of the memory array1005 described above may further include processes, features, means, orinstructions for charging the digit line of the memory cell as part ofthe read operation, wherein the time-varying signal may be applied whencharging the digit line begins.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for sensing a secondvoltage level at a node different from the digit line, wherein the latchmay be activated based at least in part on the second voltage levelsatisfying a voltage threshold. Some examples of the memory array 1005described above may further include processes, features, means, orinstructions for outputting a signal based at least in part on thesecond voltage level satisfying the voltage threshold, wherein the latchmay be activated based at least in part on the signal.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for determining thata duration of the read operation satisfies a time threshold, whereinidentifying the logic state of the memory cell may be based at least inpart on the latch not activating prior to the duration satisfying thetime threshold.

In some examples of the memory array 1005 described above, aconfiguration of the time-varying signal may be based at least in parton an expected charge of the memory cell and the first voltage level. Insome examples of the memory array 1005 described above, theconfiguration of the time-varying signal may be based at least in parton a number of logic states the memory cell may be capable of storing.

In some examples of the memory array 1005 described above, theconfiguration of the time-varying signal may be based at least in parton a number of latches used in the read operation. In some examples ofthe memory array 1005 described above, a configuration of thetime-varying signal and an interval of the time-varying signal may bepredetermined. In some examples of the memory array 1005 describedabove, a value of the time-varying signal varies in a predeterminedmanner over a predetermined interval of the time-varying signal.

The memory array 1005 may include means for sensing a first state of aferroelectric capacitor in a ferroelectric memory cell, means forsensing a second state of the ferroelectric capacitor different from thefirst state, and means for identifying a logic state of theferroelectric memory cell from at least three logic states based atleast in part on the first state and the second state. In some examples,sensing a first state of the ferroelectric capacitor and sensing asecond state of the ferroelectric capacitor may comprise sensing acombined state of the sensing capacitor. In some cases, the combinedstate may be a combination (or superposition) of a polarized state and adialectic charge state.

In some examples of the memory array 1005 described above, the firststate of the ferroelectric capacitor may be associated with apolarization of the ferroelectric capacitor. In some examples of thememory array 1005 described above, the second state of the ferroelectriccapacitor may be associated with a dielectric charge stored on theferroelectric capacitor.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for identifying afirst bit of the logic state based at least in part on the first state.Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for identifying asecond bit of the logic state based at least in part on the secondstate.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for activating atleast two latches based at least in part on a voltage level of a nodedifferent from a digit line satisfying a voltage threshold.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for applying a firsttime-varying signal to one of the at least two latches. Some examples ofthe memory array 1005 described above may further include processes,features, means, or instructions for applying a second time-varyingsignal different from the first time-varying signal to another of the atleast two latches, wherein the logic state may be identifying based atleast in part on values of the first time-varying signal and the secondtime-varying signal when activating the at least two latches.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for identifying aduration for a first voltage level of a digit line to satisfy a voltagethreshold during an access operation, the duration based at least inpart on the first state of the ferroelectric capacitor, the second stateof the ferroelectric capacitor, and a voltage applied to the digit line.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for identifying aduration for a second voltage level of a node different from a digitline to satisfy a voltage threshold, the duration based at least in parton the first state of the ferroelectric capacitor and the second stateof the ferroelectric capacitor.

The memory array 1005 may include means for activating a selectioncomponent of a ferroelectric memory cell, means for modifying a firststate of a ferroelectric capacitor of the ferroelectric memory cellbased at least in part on a voltage being applied to the ferroelectricmemory cell while the selection component is activated, means fordeactivating the selection component, and means for modifying a secondstate of the ferroelectric capacitor based at least in part on theselection component being deactivated while the voltage is applied tothe ferroelectric memory cell.

Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for activating theselection component while a plate line and a digit line coupled to theferroelectric memory cell may be grounded or virtually grounded. In someexamples of the memory array 1005 described above, modifying the firststate of the ferroelectric capacitor comprises: applying a first voltageto the ferroelectric capacitor.

In some examples of the memory array 1005 described above, applying thefirst voltage to the ferroelectric capacitor comprises: applying asecond voltage to a digit line coupled to the ferroelectric memory cell.Some examples of the memory array 1005 described above may furtherinclude processes, features, means, or instructions for applying a thirdvoltage to a plate line coupled to the ferroelectric memory cell, thethird voltage different from the second voltage, wherein the firstvoltage may be based at least in part on the second voltage and thethird voltage. In some examples of the memory array 1005 describedabove, modifying the second state of the ferroelectric capacitorcomprises: applying a fourth voltage to the ferroelectric capacitor.

In some examples of the memory array 1005 described above, applying thefourth voltage to the ferroelectric capacitor comprises: applying afifth voltage to a digit line coupled to the ferroelectric memory cell,the selection component may be deactivated while the fifth voltage isapplied to the ferroelectric memory cell, wherein the selectioncomponent may be positioned between the ferroelectric capacitor and aplate line coupled to the ferroelectric memory cell.

In some examples of the memory array 1005 described above, applying thefourth voltage to the ferroelectric capacitor comprises: applying asixth voltage to a plate line coupled to the ferroelectric memory cell,the selection component may be deactivated while the sixth voltage isapplied to the ferroelectric memory cell, wherein the selectioncomponent may be positioned between the ferroelectric capacitor and adigit line coupled to the ferroelectric memory cell.

In some examples of the memory array 1005 described above, theferroelectric memory cell may be configured to store at least threelogic states based at least in part on the first state of theferroelectric capacitor and the second state of the ferroelectriccapacitor. In some examples of the memory array 1005 described above,the first state may be a polarization state of the ferroelectriccapacitor. In some examples of the memory array 1005 described above,the second state may be a dielectric charge state of the ferroelectriccapacitor.

FIG. 11 shows a block diagram 1100 of a memory controller 1115 thatsupports time-based access of a memory cell in accordance with variousembodiments of the present disclosure. The memory controller 1115 may bean example of portions of a memory controller 1215 described withreference to FIGS. 1, 10, and 12. The memory controller 1115 may includebiasing component 1120, timing component 1125, charging component 1130,sensing manager 1135, logic determiner 1140, signal manager 1145, latchmanager 1150, cell manager 1155, timing manager 1160, and thresholdmanager 1165. Each of these modules may communicate, directly orindirectly, with one another (e.g., via one or more buses).

Biasing component 1120 may bias the digit line prior to charging thedigit line with the first voltage level.

Timing component 1125 may be configured to determine durationsassociated with read operation of a memory cell. For example, the timingcomponent may be configured to determine a duration between beginning tocharge a digit line of a memory cell and the firing of a latch. In somecases, the duration is determined based on an amount of time thatelapses between initiating a timer and the digit line charging to thefirst voltage level.

Charging component 1130 may charge a digit line coupled to a memory cellto a first voltage level, vary a current applied to the digit line overtime to charge the digit line, the time-varying current configured tomodify a time interval associated with a specific logic state of thememory cell, force a current into a ferroelectric memory cell coupled toa digit line, the ferroelectric memory cell configured to store at leastthree logic states, charge, by a current generator, the digit line tothe first voltage level based on applying the current, and charge thedigit line of the memory cell as part of the read operation, where thetime-varying signal is applied when charging the digit line begins. Insome cases, the first voltage level to which the digit line is chargedis a predetermined voltage level based on at least on one of a set ofpossible charge states of the memory cell. In some cases, the digit lineis charged by a cascode coupled to the digit line and to a sensecomponent. In some cases, the current is forced based on performing aread operation on the ferroelectric memory cell. In some cases, thecurrent is forced by a current generator coupled to the digit line andthe node.

Sensing manager 1135 may determine a duration for the digit line tocharge to the first voltage level, sense a voltage at a node differentfrom the digit line, the voltage based on a first voltage level of thedigit line, output, by a sense component, a signal based on the voltagesatisfying the voltage threshold, sense a second voltage level at a nodedifferent from the digit line, where the latch is activated based on thesecond voltage level satisfying a voltage threshold, output a signalbased on the second voltage level satisfying the voltage threshold,where the latch is activated based on the signal, sense a first state ofa ferroelectric capacitor in a ferroelectric memory cell, and sense asecond state of the ferroelectric capacitor different from the firststate.

Logic determiner 1140 may identify a logic state of the memory cellbased on the duration for the digit line to reach the first voltagelevel, identify a value of a time-varying signal based on the duration,where the logic state is based on the value of the time-varying signal,identify a first bit of the logic state after a second duration lessthan the duration, identify a second bit of the logic state after theduration, identify a logic state of the ferroelectric memory cell fromthe at least three logic states based on the voltage satisfying avoltage threshold, identify the logic state of the ferroelectric memorycell is based on a polarization state of a ferroelectric capacitor ofthe ferroelectric memory cell and a charge state of the ferroelectriccapacitor of the ferroelectric memory cell, identify a logic state ofthe memory cell based on a value of the time-varying signal present atthe latch when the latch is activated, identify a logic state of theferroelectric memory cell from at least three logic states based on thefirst state and the second state, identify a first bit of the logicstate based on the first state, and identify a second bit of the logicstate based on the second state.

Signal manager 1145 may apply a time-varying signal to a latch afterinitiating a read operation on a memory cell, apply a secondtime-varying signal to a second latch based on performing the readoperation on the memory cell, the second time-varying signal differentfrom the time-varying signal, the second latch different from the latch,apply a first time-varying signal to one of the at least two latches,and apply a second time-varying signal different from the firsttime-varying signal to another of the at least two latches, where thelogic state is identifying based on values of the first time-varyingsignal and the second time-varying signal when activating the at leasttwo latches. In some cases, a configuration of the second time-varyingsignal is based on a configuration of the time-varying signal, where thetime-varying signal and the second time-varying signal cooperate todefine at least three logic states. In some cases, a configuration ofthe time-varying signal is based on an expected charge of the memorycell and the first voltage level. In some cases, the configuration ofthe time-varying signal is based on a number of logic states the memorycell is capable of storing. In some cases, the configuration of thetime-varying signal is based on a number of latches used in the readoperation. In some cases, a configuration of the time-varying signal andan interval of the time-varying signal is predetermined. In some cases,a value of the time-varying signal varies in a predetermined manner overa predetermined interval of the time-varying signal.

Latch manager 1150 may activate a first latch based on the voltagesatisfying the voltage threshold, activate a second latch different fromthe first latch based on the voltage satisfying the voltage threshold,apply a first time-varying signal to the first latch, apply a secondtime-varying signal to the second latch different from the firsttime-varying signal, where the logic state of the ferroelectric memorycell is based on values of the first time-varying signal and the secondtime-varying signal when the first latch and the second latch areactivated, activate the latch based on a digit line that is coupled tothe memory cell charging to a first voltage level as part of the readoperation, activate the second latch based on the digit line that iscoupled to the memory cell charging to the first voltage level, whereidentifying the logic state of the memory cell is based on thetime-varying signal present at the latch and the second time-varyingsignal present at the second latch when the latch and the second latchare activated, and activate at least two latches based on a voltagelevel of a node different from a digit line satisfying a voltagethreshold.

Cell manager 1155 may select the memory cell from a set of memory cellsbased on a read operation being initiated, activate a selectioncomponent of the ferroelectric memory cell, where the current is forcedbased on the selection component being activated, activate a selectioncomponent of the ferroelectric memory cell while a plate line and thedigit line coupled to the ferroelectric memory cell are grounded orvirtually grounded, perform a write-back operation on the ferroelectricmemory cell based on the identified logic state of the ferroelectricmemory cell, isolate the memory cell from the latch after the digit linecharges to the first voltage as part of the read operation, modify afirst state of a ferroelectric capacitor of the ferroelectric memorycell based on a voltage being applied to the ferroelectric memory cellwhile the selection component is activated, deactivate the selectioncomponent, modify a second state of the ferroelectric capacitor based onthe selection component being deactivated while the voltage is appliedto the ferroelectric memory cell, activate the selection component whilea plate line and a digit line coupled to the ferroelectric memory cellare grounded or virtually grounded, apply a third voltage to a plateline coupled to the ferroelectric memory cell, the third voltagedifferent from the second voltage, where the first voltage is based onthe second voltage and the third voltage, and activate a selectioncomponent of a ferroelectric memory cell.

In some cases, the second state is a dielectric charge state of theferroelectric capacitor. In some cases, the memory cell includes aferroelectric capacitor. In some cases, the memory cell includes adielectric capacitor. In some cases, the memory cell is configured tostore at least three logic states. In some cases, the memory cell isconfigured to store two logic states. In some cases, the memory cell isconfigured to store at least three logic states. In some cases, theidentified logic state of the memory cell is selected from the at leastthree logic states. In some cases, the first state of the ferroelectriccapacitor is associated with a polarization of the ferroelectriccapacitor.

In some cases, the second state of the ferroelectric capacitor isassociated with a dielectric charge stored on the ferroelectriccapacitor. In some cases, the duration is based on a stable state of acapacitor of the memory cell and a volatile state of the capacitor ofthe memory cell. In some cases, modifying the first state of theferroelectric capacitor includes: applying a first voltage to theferroelectric capacitor. In some cases, applying the first voltage tothe ferroelectric capacitor includes: applying a second voltage to adigit line coupled to the ferroelectric memory cell. In some cases,modifying the second state of the ferroelectric capacitor includes:applying a fourth voltage to the ferroelectric capacitor. In some cases,applying the fourth voltage to the ferroelectric capacitor includes:applying a fifth voltage to a digit line coupled to the ferroelectricmemory cell, the selection component being deactivated while the fifthvoltage is applied to the ferroelectric memory cell, where the selectioncomponent is positioned between the ferroelectric capacitor and a plateline coupled to the ferroelectric memory cell. In some cases, applyingthe fourth voltage to the ferroelectric capacitor includes: applying asixth voltage to a plate line coupled to the ferroelectric memory cell,the selection component being deactivated while the sixth voltage isapplied to the ferroelectric memory cell, where the selection componentis positioned between the ferroelectric capacitor and a digit linecoupled to the ferroelectric memory cell. In some cases, theferroelectric memory cell is configured to store at least three logicstates based on the first state of the ferroelectric capacitor and thesecond state of the ferroelectric capacitor. In some cases, the firststate is a polarization state of the ferroelectric capacitor.

Timing manager 1160 may initiate a timer based on performing a readoperation on the memory cell, where the duration is determined based onthe timer, identify a duration for a second voltage level present on thenode to satisfy the voltage threshold, where the logic state isidentified based on the duration, determine that a duration of the readoperation satisfies a time threshold, where identifying the logic stateof the memory cell is based on the latch not activating prior to theduration satisfying the time threshold, identify a duration for a firstvoltage level of a digit line to satisfy a voltage threshold during anaccess operation, the duration based on the first state of theferroelectric capacitor, the second state of the ferroelectriccapacitor, and a voltage applied to the digit line, and identify aduration for a second voltage level of a node different from a digitline to satisfy a voltage threshold, the duration based on the firststate of the ferroelectric capacitor and the second state of theferroelectric capacitor. In some cases, the duration is based on a totalcharge stored on a ferroelectric capacitor of the ferroelectric memorycell. In some cases, the total charge includes a volatile charge of theferroelectric capacitor and a stable charge of the ferroelectriccapacitor.

Threshold manager 1165 may determine that a second voltage level at anode different from the digit line satisfies a voltage threshold, wherethe duration is based on the second voltage level satisfying the voltagethreshold and sense, by a sense component, the second voltage level atthe node, where the logic state is identified based on the secondvoltage level. In some cases, the voltage threshold is less than abiasing voltage used to generate a stable state of the memory cell.

FIG. 12 shows a diagram of a system 1200 including a device 1205 thatsupports time-based access of a memory cell in accordance with variousembodiments of the present disclosure. Device 1205 may be an example ofor include the components of memory controller 1015 as described above,e.g., with reference to FIG. 10. Device 1205 may include components forbi-directional voice and data communications including components fortransmitting and receiving communications, including memory controller1215, memory cells 1220, basic input/output system (BIOS) component1225, processor 1230, I/O controller 1235, and peripheral components1240. These components may be in electronic communication via one ormore busses (e.g., bus 1210). Memory cells 1220 may store information(i.e., in the form of a logical state) as described herein.

BIOS component 1225 be a software component that includes BIOS operatedas firmware, which may initialize and run various hardware components.BIOS component 1225 may also manage data flow between a processor andvarious other components, e.g., peripheral components, input/outputcontrol component, etc. BIOS component 1225 may include a program orsoftware stored in read only memory (ROM), flash memory, or any othernon-volatile memory.

Processor 1230 may include an intelligent hardware device, (e.g., ageneral-purpose processor, a DSP, a central processing unit (CPU), amicrocontroller, an ASIC, an FPGA, a programmable logic device, adiscrete gate or transistor logic component, a discrete hardwarecomponent, or any combination thereof). In some cases, processor 1230may be configured to operate a memory array using a memory controller.In other cases, a memory controller may be integrated into processor1230. Processor 1230 may be configured to execute computer-readableinstructions stored in a memory to perform various functions (e.g.,functions or tasks supporting time-based access of a memory cell).

I/O controller 1235 may manage input and output signals for device 1205.I/O controller 1235 may also manage peripherals not integrated intodevice 1205. In some cases, I/O controller 1235 may represent a physicalconnection or port to an external peripheral. In some cases, I/Ocontroller 1235 may utilize an operating system such as iOS®, ANDROID®,MS-DOS®, MS-WINDOWS®, OS/2®, UNIX®, LINUX®, or another known operatingsystem. In other cases, I/O controller 1235 may represent or interactwith a modem, a keyboard, a mouse, a touchscreen, or a similar device.In some cases, I/O controller 1235 may be implemented as part of aprocessor. In some cases, a user may interact with device 1205 via I/Ocontroller 1235 or via hardware components controlled by I/O controller1235.

Peripheral components 1240 may include any input or output device, or aninterface for such devices. Examples may include disk controllers, soundcontroller, graphics controller, Ethernet controller, modem, universalserial bus (USB) controller, a serial or parallel port, or peripheralcard slots, such as peripheral component interconnect (PCI) oraccelerated graphics port (AGP) slots.

FIG. 13 shows a flowchart illustrating a method 1300 for time-basedaccess of a memory cell in accordance with various embodiments of thepresent disclosure. The operations of method 1300 may be implemented bya memory controller 1015 or its components as described herein. Forexample, the operations of method 1300 may be performed by a memorycontroller as described with reference to FIGS. 10 through 12. In someexamples, a memory controller 1015 may execute a set of codes to controlthe functional elements of the device to perform the functions describedbelow. Additionally or alternatively, the memory controller 1015 mayperform portions of the functions described below using special-purposehardware.

At block 1305 the memory controller 1015 may charge a digit line coupledto a memory cell to a first voltage level. The operations of block 1305may be performed according to the methods described with reference toFIGS. 1 through 9. In certain examples, portions of the operations ofblock 1305 may be performed by a charging component as described withreference to FIGS. 10 through 12.

At block 1310 the memory controller 1015 may determine a duration forthe digit line to charge to the first voltage level. The operations ofblock 1310 may be performed according to the methods described withreference to FIGS. 1 through 9. In certain examples, portions of theoperations of block 1310 may be performed by a sensing manager asdescribed with reference to FIGS. 10 through 12.

At block 1315 the memory controller 1015 may identify a logic state ofthe memory cell based at least in part on the duration for the digitline to reach the first voltage level. The operations of block 1315 maybe performed according to the methods described with reference to FIGS.1 through 9. In certain examples, portions of the operations of blockmay be performed by a logic determiner as described with reference toFIGS. 10 through 12.

In some cases, the memory cell is configured to store two logic states.In some cases, the duration is determined based at least in part on anamount of time that elapses between initiating a timer and the digitline charging to the first voltage level. In some cases, the voltagethreshold is less than a biasing voltage used to generate a stable stateof the memory cell. In some cases, the first voltage level to which thedigit line is charged is a predetermined voltage level based at least inpart on at least on one of a plurality of possible charge states of thememory cell. In some cases, the digit line is charged by a cascodecoupled to the digit line and to a sense component. In some cases, theduration is based at least in part on a stable state of a capacitor ofthe memory cell and a volatile state of the capacitor of the memorycell. In some cases, the memory cell includes a ferroelectric capacitor.In some cases, the memory cell includes a dielectric capacitor. In somecases, the memory cell is configured to store at least three logicstates.

FIG. 14 shows a flowchart illustrating a method 1400 for time-basedaccess of a memory cell in accordance with various embodiments of thepresent disclosure. The operations of method 1400 may be implemented bya memory controller 1015 or its components as described herein. Forexample, the operations of method 1400 may be performed by a memorycontroller as described with reference to FIGS. 10 through 12. In someexamples, a memory controller 1015 may execute a set of codes to controlthe functional elements of the device to perform the functions describedbelow. Additionally or alternatively, the memory controller 1015 mayperform portions of the functions described below using special-purposehardware.

At block 1405 the memory controller 1015 may force a current into aferroelectric memory cell coupled to a digit line, the ferroelectricmemory cell configured to store at least three logic states. Theoperations of block 1405 may be performed according to the methodsdescribed with reference to FIGS. 1 through 9. In certain examples,portions of the operations of block 1405 may be performed by a chargingcomponent as described with reference to FIGS. 10 through 12.

At block 1410 the memory controller 1015 may sense a voltage at a nodedifferent from the digit line, the voltage based at least in part on afirst voltage level of the digit line. The operations of block 1410 maybe performed according to the methods described with reference to FIGS.1 through 9. In certain examples, portions of the operations of block1410 may be performed by a sensing manager as described with referenceto FIGS. 10 through 12.

At block 1415 the memory controller 1015 may identify a logic state ofthe ferroelectric memory cell from the at least three logic states basedat least in part on the voltage satisfying a voltage threshold. Theoperations of block 1415 may be performed according to the methodsdescribed with reference to FIGS. 1 through 9. In certain examples,portions of the operations of block 1415 may be performed by a logicdeterminer as described with reference to FIGS. 10 through 12.

In some cases, the memory controller 1015 may identify a duration for asecond voltage level present on the node to satisfy the voltagethreshold, where the logic state is identified based at least in part onthe duration. In some cases, the current is forced by a currentgenerator coupled to the digit line and the node. In some cases, theduration is based at least in part on a total charge stored on aferroelectric capacitor of the ferroelectric memory cell. In some cases,the total charge comprises a volatile charge of the ferroelectriccapacitor and a stable charge of the ferroelectric capacitor. In somecases, the current is forced based at least in part on performing a readoperation on the ferroelectric memory cell.

It should be noted that the methods described above describe possibleimplementations, and that the operations and the steps may be rearrangedor otherwise modified and that other implementations are possible.Furthermore, embodiments from two or more of the methods may becombined.

Information and signals described herein may be represented using any ofa variety of different technologies and techniques. For example, data,instructions, commands, information, signals, bits, symbols, and chipsthat may be referenced throughout the above description may berepresented by voltages, currents, electromagnetic waves, magneticfields or particles, optical fields or particles, or any combinationthereof. Some drawings may illustrate signals as a single signal;however, it will be understood by a person of ordinary skill in the artthat the signal may represent a bus of signals, where the bus may have avariety of bit widths.

As used herein, the term “virtual ground” refers to a node of anelectrical circuit that is held at a voltage of approximately zero volts(0V) but that is not directly connected with ground. Accordingly, thevoltage of a virtual ground may temporarily fluctuate and return toapproximately 0V at steady state. A virtual ground may be implementedusing various electronic circuit elements, such as a voltage dividerconsisting of operational amplifiers and resistors. Otherimplementations are also possible. “Virtual grounding” or “virtuallygrounded” means connected to approximately 0V.

The term “electronic communication” and “coupled” refer to arelationship between components that support electron flow between thecomponents. This may include a direct connection between components ormay include intermediate components. Components in electroniccommunication or coupled to one another may be actively exchangingelectrons or signals (e.g., in an energized circuit) or may not beactively exchanging electrons or signals (e.g., in a de-energizedcircuit) but may be configured to and operable to exchange electrons orsignals upon a circuit being energized. By way of example, twocomponents physically connected via a switch (e.g., a transistor) are inelectronic communication or may be coupled regardless of the state ofthe switch (i.e., open or closed).

As used herein, the term “substantially” means that the modifiedcharacteristic (e.g., a verb or adjective modified by the termsubstantially) need not be absolute but is close enough so as to achievethe advantages of the characteristic.

As used herein, the term “electrode” may refer to an electricalconductor, and in some cases, may be employed as an electrical contactto a memory cell or other component of a memory array. An electrode mayinclude a trace, wire, conductive line, conductive layer, or the likethat provides a conductive path between elements or components of memoryarray 100.

The term “isolated” refers to a relationship between components in whichelectrons are not presently capable of flowing between them; componentsare isolated from each other if there is an open circuit between them.For example, two components physically connected by a switch may beisolated from each other when the switch is open.

As used herein, the term “shorting” refers to a relationship betweencomponents in which a conductive path is established between thecomponents via the activation of a single intermediary component betweenthe two components in question. For example, a first component shortedto a second component may exchange electrons with the second componentwhen a switch between the two components is closed. Thus, shorting maybe a dynamic operation that enables the flow of charge betweencomponents (or lines) that are in electronic communication.

The devices discussed herein, including memory array 100, may be formedon a semiconductor substrate, such as silicon, germanium,silicon-germanium alloy, gallium arsenide, gallium nitride, etc. In somecases, the substrate is a semiconductor wafer. In other cases, thesubstrate may be a silicon-on-insulator (SOI) substrate, such assilicon-on-glass (SOG) or silicon-on-sapphire (SOP), or epitaxial layersof semiconductor materials on another substrate. The conductivity of thesubstrate, or sub-regions of the substrate, may be controlled throughdoping using various chemical species including, but not limited to,phosphorous, boron, or arsenic. Doping may be performed during theinitial formation or growth of the substrate, by ion-implantation, or byany other doping means.

A transistor or transistors discussed herein may represent afield-effect transistor (FET) and comprise a three terminal deviceincluding a source, drain, and gate. The terminals may be connected toother electronic elements through conductive materials, e.g., metals.The source and drain may be conductive and may comprise a heavily-doped,e.g., degenerate, semiconductor region. The source and drain may beseparated by a lightly-doped semiconductor region or channel. If thechannel is n-type (i.e., majority carriers are electrons), then the FETmay be referred to as a n-type FET. If the channel is p-type (i.e.,majority carriers are holes), then the FET may be referred to as ap-type FET. The channel may be capped by an insulating gate oxide. Thechannel conductivity may be controlled by applying a voltage to thegate. For example, applying a positive voltage or negative voltage to ann-type FET or a p-type FET, respectively, may result in the channelbecoming conductive. A transistor may be “on” or “activated” when avoltage greater than or equal to the transistor's threshold voltage isapplied to the transistor gate. The transistor may be “off” or“deactivated” when a voltage less than the transistor's thresholdvoltage is applied to the transistor gate.

The description set forth herein, in connection with the appendeddrawings, describes example configurations and does not represent allthe examples that may be implemented or that are within the scope of theclaims. The term “exemplary” used herein means “serving as an example,instance, or illustration,” and not “preferred” or “advantageous overother examples.” The detailed description includes specific details forthe purpose of providing an understanding of the described techniques.These techniques, however, may be practiced without these specificdetails. In some instances, well-known structures and devices are shownin block diagram form in order to avoid obscuring the concepts of thedescribed examples.

In the appended figures, similar components or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If just the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

Information and signals described herein may be represented using any ofa variety of different technologies and techniques. For example, data,instructions, commands, information, signals, bits, symbols, and chipsthat may be referenced throughout the above description may berepresented by voltages, currents, electromagnetic waves, magneticfields or particles, optical fields or particles, or any combinationthereof.

The various illustrative blocks and modules described in connection withthe disclosure herein may be implemented or performed with ageneral-purpose processor, a DSP, an ASIC, an FPGA or other programmablelogic device, discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed herein. A general-purpose processor may be a microprocessor,but in the alternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices (e.g., a combinationof a digital signal processor (DSP) and a microprocessor, multiplemicroprocessors, one or more microprocessors in conjunction with a DSPcore, or any other such configuration).

The functions described herein may be implemented in hardware, softwareexecuted by a processor, firmware, or any combination thereof. Ifimplemented in software executed by a processor, the functions may bestored on or transmitted over as one or more instructions or code on acomputer-readable medium. Other examples and implementations are withinthe scope of the disclosure and appended claims. For example, due to thenature of software, functions described above can be implemented usingsoftware executed by a processor, hardware, firmware, hardwiring, orcombinations of any of these. Features implementing functions may alsobe physically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations. Also, as used herein, including in the claims, “or” as usedin a list of items (for example, a list of items prefaced by a phrasesuch as “at least one of” or “one or more of”) indicates an inclusivelist such that, for example, a list of at least one of A, B, or C meansA or B or C or AB or AC or BC or ABC (i.e., A and B and C). Also, asused herein, the phrase “based on” shall not be construed as a referenceto a closed set of conditions. For example, an exemplary step that isdescribed as “based on condition A” may be based on both a condition Aand a condition B without departing from the scope of the presentdisclosure. In other words, as used herein, the phrase “based on” shallbe construed in the same manner as the phrase “based at least in parton.”

Computer-readable media includes both non-transitory computer storagemedia and communication media including any medium that facilitatestransfer of a computer program from one place to another. Anon-transitory storage medium may be any available medium that can beaccessed by a general purpose or special purpose computer. By way ofexample, and not limitation, non-transitory computer-readable media cancomprise RAM, ROM, electrically erasable programmable read only memory(EEPROM), compact disk (CD) ROM or other optical disk storage, magneticdisk storage or other magnetic storage devices, or any othernon-transitory medium that can be used to carry or store desired programcode means in the form of instructions or data structures and that canbe accessed by a general-purpose or special-purpose computer, or ageneral-purpose or special-purpose processor.

Also, any connection is properly termed a computer-readable medium. Forexample, if the software is transmitted from a website, server, or otherremote source using a coaxial cable, fiber optic cable, twisted pair,digital subscriber line (DSL), or wireless technologies such asinfrared, radio, and microwave, then the coaxial cable, fiber opticcable, twisted pair, digital subscriber line (DSL), or wirelesstechnologies such as infrared, radio, and microwave are included in thedefinition of medium. Disk and disc, as used herein, include CD, laserdisc, optical disc, digital versatile disc (DVD), floppy disk andBlu-ray disc where disks usually reproduce data magnetically, whilediscs reproduce data optically with lasers. Combinations of the aboveare also included within the scope of computer-readable media.

The description herein is provided to enable a person skilled in the artto make or use the disclosure. Various modifications to the disclosurewill be readily apparent to those skilled in the art, and the genericprinciples defined herein may be applied to other variations withoutdeparting from the scope of the disclosure. Thus, the disclosure is notlimited to the examples and designs described herein, but is to beaccorded the broadest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. An electronic memory apparatus, comprising: amemory cell in electronic communication with a digit line and operableto store a plurality of logic states; a current generator coupled to thedigit line and operable to charge the digit line to a voltage level; alatch operable to output a value of a time-varying signal based at leastin part on the current generator charging the digit line to the voltagelevel; and a controller coupled to the latch and operable to identify alogic state of the memory cell based at least in part on the value ofthe time-varying signal received from the latch.
 2. The electronicmemory apparatus of claim 1, further comprising: a sense componentcoupled to a node of the current generator different than the digitline, the sense component operable to detect a voltage present on thenode, wherein the voltage is based at least in part on the voltage levelof the digit line.
 3. The electronic memory apparatus of claim 2,wherein the latch is coupled to the sense component and is operable tooutput the value of the time-varying signal based at least in part onreceiving a signal from the sense component.
 4. The electronic memoryapparatus of claim 2, wherein: the sense component comprises aninverter.
 5. The electronic memory apparatus of claim 1, wherein thecurrent generator is operable to apply a time-varying current to thedigit line to charge the digit line to the voltage level, wherein thevalue of the time-varying signal output by the latch is based at leastin part on applying the time-varying current.
 6. An electronic memoryapparatus of claim 1, further comprising: a second latch coupled to thecontroller and operable to output a value of a second time-varyingsignal based at least in part on the current generator charging thedigit line to the voltage level.
 7. The electronic memory apparatus ofclaim 6, wherein the controller is operable to identify the logic stateof the memory cell based at least in part on the value of thetime-varying signal received from the latch and the value of the secondtime-varying signal received from the second latch.
 8. The electronicmemory apparatus of claim 1, wherein the controller is operable to biasthe digit line prior to the digit line being charged by the currentgenerator.
 9. The electronic memory apparatus of claim 1, wherein thecontroller is operable to identify a first bit of the logic state storedin the memory cell after a first duration and identify a second bit ofthe logic state stored in the memory cell after a second duration. 10.The electronic memory apparatus of claim 1, wherein: the currentgenerator comprises a cascode.
 11. The electronic memory apparatus ofclaim 1, wherein: the plurality of logic states comprises a plurality ofstable states and a plurality of volatile states.
 12. The electronicmemory apparatus of claim 1, wherein the memory cell is operable to beoperated as one of: a volatile memory cell, a non-volatile memory cell,or a partly volatile and a partly non-volatile memory cell.
 13. Amethod, comprising: charging, using a time-varying current as part of aread operation, a digit line coupled to a memory cell; identifying afirst bit of a logic state of the memory cell based at least in part ona first duration for charging a voltage of the digit line to a firstvoltage level; and identifying a second bit of the logic state based atleast in part on a second duration for charging the voltage of the digitline to a second voltage level.
 14. The method of claim 13, furthercomprising: initiating a timer based at least in part on performing theread operation on the memory cell, wherein the first duration or thesecond duration is determined based at least in part on an amount oftime that elapses between initiating the timer and the digit linecharging to the first voltage level.
 15. The method of claim 13, furthercomprising: determining that a third voltage level at a node differentfrom the digit line satisfies a voltage threshold, wherein the firstduration is based at least in part on the third voltage level satisfyingthe voltage threshold.
 16. The method of claim 13, further comprising:identifying a first value of a time-varying signal at or after a firsttime when the voltage charges to the first voltage level, whereinidentifying the first bit of the logic state is based at least in parton the first value of the time-varying signal at or after the firsttime; and identifying a second value of the time-varying signal at orafter a second time when the voltage charges to the second voltagelevel, wherein identifying the second bit of the logic state is based atleast in part on the second value of the time-varying signal at or afterthe second time.
 17. The method of claim 13, further comprising: biasingthe digit line prior to charging the digit line.
 18. A method,comprising: charging, using a time-varying current as part of a readoperation, a digit line coupled to a memory cell; determining a timethat a voltage level associated with the digit line satisfies athreshold based at least in part on charging the digit line;identifying, at or after the time that the voltage level satisfies thethreshold, a first value of a first time-varying signal and a secondvalue of a second time-varying signal; and identifying a logic state ofthe memory cell based at least in part on the first value of the firsttime-varying signal and the second value of the second time-varyingsignal.
 19. The method of claim 18, further comprising: initiating thefirst time-varying signal and the second time-varying signal based atleast in part on charging the digit line, wherein identifying the firstvalue and the second value at or after the time is based at least inpart on initiating the first time-varying signal and the secondtime-varying signal.
 20. The method of claim 18, further comprising:determining a second time that a second voltage level at a nodedifferent than the digit line satisfies a second threshold based atleast in part on charging the digit line, wherein identifying the firstvalue and the second value at or after the time is based at least inpart on the second voltage level satisfying the second threshold.